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The evaluation of stainless steels for condenser tube nest construction
Corrosion Science, 1972, Vol. 12, pp. 247 to 264. Pergamon Press. Printed in Great Bfitaln
THE EVALUATION OF STAINLESS STEELS FOR CONDENSER TUBE NEST CONSTRUCTION* D. W. C. BAKER, W. E. HEATON~" a n d B. C. PATIENT Central Electricity Generating Board, Scientific Services Department, West Farm Place, Chalk Lane, Cockfosters, Barnet, Herts, England Abstract--The possibility of using austenitic stainless steel for condenser tube nest construction for CEGB plant was investigated by laboratory and field trials. It was found that both the Mo bearing (AISI Type 316 and 316L) steels and the straight 18% Cr-10%Ni (AISI Type 304 and 304L) steels suffered crevice and pitting corrosion on exposure to sea and estuarine cooling waters and cannot be recommended for service in the U.K. under these conditions. R6sum6---Ona 6tudi6 en laboratoire et en usine la possibilit6 d'utiliser des aciers inoxydables aust6nitiques pour la confection de tubes de condenseurs pour l'usine de la CEBG. On a trouv6 que les aciers des types AISI 316 et 316 L au molybd6ne et les aciers de type AISI 304 et 304 L it 18% de chrome et 10% de nickel subissent tous deux une corrosion caverneuse et par piqfires en cas d'exposition gt des eaux de refroidissement marine et d'estuaire, et que leur utilisation ne peut pas 6tre recommand6e dans le Royaume Uni dans de telles conditions. Zusammenfassung--Die M6glichkeit, austenitischen rostfreien Stahl fiir die Kondensatorrol~rbiJndelKonstruktion einer CEGB-Anlage zu verwenden, wurde im Laboratorium und praktisch untersucht. Es wurde festgestellt, dass die Mo-haltigen St~hle (AISI Type 316 und 316L) und die St~ihle mit 18%Cr und I0%Ni (AISI Type 304 und 304L) durch Einwirkung von Ktihlwasser aus dem Meer und aus FlussmiJndungen Spalten- und Narbenkorrosion erlitten und zur Verwendung in Grossbritannien unter solchen Bedingungen nicht empfohlen werden kfnnen. INTR.ODUCTION THE POSSIBILITY of using stainless steels for the c o n s t r u c t i o n of t u r b i n e condenser tube nests for C E G B plant was considered for the following reasons: (a) T o eliminate a potential source of c o n t a m i n a t i o n of the condensate a n d hence the feedwater by C u a n d other metals leached from c o n v e n t i o n a l non-ferrous tube alloys. (b) T o improve the integrity of condensers by permitting welding o f tube/tubeplate joints. (c) To replace the established tube alloys where technical or economic advantages could be shown. The problems arising from C u c o n t a m i n a t i o n of the water entering the boiler, including the deposition of C u oxides in the turbine, are intensified in " o n c e - t h r o u g h " supercritical pressure systems. 1,2 At the time of starting the present investigation (1965), it was proposed that the 660 m W units to be installed at Drax (Yorkshire) would operate at supercritical pressure and there was an immediate need to eliminate C u bearing materials from the water circuit. It was not k n o w n whether the c o n d e n s e r *Manuscript received 11 May 1971 ; in revised form 24 June 1971. tCentral Electricity Generating Board, Scientific Services Department, Sea-water Corrosion Laboratory, Brighton Power Station, Portslade, Brighton, Sussex, BN4 lWG. 247
D. W. C.
248 TABLE 1.
BAKER,
W. E. HEATONand B. C. PATIENT
VARIATION OF FEED AND BOILER DRUM WATER SPECIFICATIONS WITH UNIT SIZE AND STEAM CONDITIONS
Steam conditions
6"25 N/mm z, 485°C 900 lb/in. °(905°F)
10-4 N/mm -~, 565°C "1500 lb/in.'-" (1050°F)
Commissioning date and size
1952 60 mW
1957 120 mW
16.2 N/ram ~-,565'~C 2350 Ib/in. 2 (1050°F) 1963 200 mW
1967 500 mW
Feed water
Conductivity-micromho/cm Dissolved oxygen --ppm Metals--ppm pH value
~ 1.6
:b 0"5 ~r 0'3 (after cation exchange to hydrogen chloride)
> 0.02 Not specified
:b 0.007 (Fe + Cu):l- 0'01
:b 0.007 (Fe-I- Cu):b 0.01
8.0-8.5
Not specified directly
8.8-9.2 at 25°C
:b 33 as NaC1 ::1- 3.0 as SiO~
21" 16 as NaCI 21- 0-3 as SiO~
:~ 0'3 21, 0.007 (Fe+Cu +Ni):b 0.01 8'5 at 25°C
Boiler drum water
Chloride--ppm Silica--ppm
> 50 as NaCI :b 6-0 as SiO.~
> 4 as NaCI :b 0.2 as SiO2
would be, in fact, a significant source o f Cu pick-up and work was also initiated to cover this aspect. In the event it was decided to install subcritical pressure drum units at Drax and the need for alternative condenser materials diminished. However, the prospect o f future advanced cycle A G R stations, located on the coast, maintained interest in the future application of stainless steels in a marine environment. The increasing stringency of feedwater purity requirements (Table 1) for advanced units also emphasized the need for improved integrity o f condensers. Mechanical methods o f tube fixing using expansion or packing do not eliminate potential interface leak paths, and welding appears to offer an alternative to the use of double tubeplates, which present their own problems and economic penalties. The belief that austenitic stainless steels would assist in this respect was encouraged by the knowledge that they offer much better welding characteristics than the non-ferrous alloys. The continuing corrosion and erosion problems experienced with non-ferrous alloys in aggressive sea-water and estuarine environments also warranted the investigation of alternative materials. Stainless steels were used initially in the U.S.A. for condenser tubing to overcome specific problems in the steam impingement and incondensible gas exhaust zones o f condensers. 3 Successful experience led to their wider use as replacements for established non-ferrous alloys where cooling water corrosion problems were met. 4 It has been stated that the majority o f new condensers in the U.S.A. are now partially or entirely tubed with austenitic stainless steel. 5 However, the cost o f austenitic steel tubing relative to that o f brass in the U.S.A. may be the dominant factor. Interest in the possibility of using stainless steels to replace Cu alloys has also been stimulated in Eastern 6 and Western Europe. 7 The work reported here was part o f a collaborative programme o f evaluation carried out with major U.K. turbine-makers, namely: English Electric Co. Ltd., Associated Electrical Industries Ltd., and C. A. Parsons Ltd.
The evaluation of stainless steels
249
MATERIALS INVESTIGATED The cost of austenitic steel tubes made by tungsten electrode inert gas (TIG) welding of preformed rolled strip is significantly less than that of seamless tubing produced by drawing from forged or extruded hollows. For this reason, the evaluation was carried out on welded tubing produced in this manner. While tubing can be produced to the required dimensional tolerances s in the "as welded" condition it was recommended by the manufacturers, following American practice, that after welding the tubing should be annealed and then cold-drawn and reannealed to ensure that the weld zone is completely recrystallized. Each of the additional processes adds significantly to the final cost. In the present investigation "as welded", "welded and annealed" and "welded, annealed, drawn and reannealed" tubing was examined because, apart from the economic advantages of minimizing post-welding treatments, the behaviour of the seam welds would give guidance on the likely performance of welded tube/tubeplate joints which it would not be practicable to work or heat-treat after welding. Four different austenitic stainless steels were selected for investigation, covering the "normal" and "low" C varieties of both the straight Cr-Ni and the Mo bearing alloys: °'oCr
%Ni
?'oMo
%C
Type 304
18/20
8/12
--
0.08 maximum
Type 304L
18/20
8/12
--
0.03
,,
Type 316
16/18
10/14
2/3
0-08
,,
Type 316L
16/18
10/14
2/3
0-03
,,
Tile plain Cr-Ni steels (Type 304 and 304L) are claimed to be satisfactory in nonaggressive waters but the more expensive Mo steels (Type 316 and 316L) are required for brackish or sea-waters. Steels of the "low" C type were included because they offer greater resistance to intercrystalline corrosion of the heat affected zones of welds. They are, however, more costly than the corresponding "normal" C varieties. Apart from the obvious economic implications, the investigation of a range of steels of differentcomposition and treatment offered the chance, if they showed different responses, of characterizing the various test environments and defining the limits of suitability of each category. To conform with existing practice, 25.4 mm o.dia, tubing was used for the investigation, but the high strength and modulus of the steels compared with nonferrous tube alloys allowed a reduction in thickness of the tubing compared with the non-ferrous materials to be considered. For field trials in operating condensers 18 SWG (0.048 in., 1"2 mm) tubing was used to match the other tubes in the nest but both 20 SWG (0.036 in., 0.9 ram) and 22 SWG (0.028 in., 0.7 mm) tubing was used for the other parts of the programme, including the welding trials, since they appeared to be feasible for condenser construction from a mechanical standpoint. To summarize, the various categories of steel tubing which were evaluated are listed below; in all cases the analyses conformed to specification.
250
D.W.C. BAKER,W. E. HEATONand B. C. PATIENT
Austenitic Type 304 ,, ,, ,, ,,
Type 316 Type 316L Type 304 Type 304L
,, ,, ,, ,, ,, ,,
Type 316 Type 316L Type 304 Type 304L Type 316 Type 316L
as welded (subsequently referred to as 304 W) (typical composition 0.06%C, 18.2%Cr, 9.5%Ni) as welded (316 W) (0.05%C, 17.4%Cr, 11.4%Ni, 2.5%Mo) as welded (316L W) (0.02%C, 17.4%Cr, 12.6%Ni, 2.5%Mo) welded and'annealed (304 WA) welded and annealed (304LWA) (0.03%C, 17.3%Cr, 12%Ni) welded and annealed (316 WA) welded and annealed (316L WA) welded, annealed, drawn and annealed (304 WADA) welded, annealed, drawn and annealed (304L WADA) welded, annealed, drawn and annealed (316 WADA) welded, annealed, drawn and annealed (316L WADA).
For those parts of the programme requiring tubeplate materials, e.g. the welding trials, plate or bar material of matching specification was used as far as possible. EVALUATION PROGRAMME The complete programme covered the following aspects: (a) Metallurgical examination of tubing as supplied. (b) Development of tube/tubeplate welding techniques. This work was undertaken by North Western Region Scientific Services Department, CEGB, at their Didsbury Laboratories. (c) Replacement of non-ferrous tubing in operating condensers using various types of cooling water. The stainless steel tubes were fixed by conventional means into the existing naval brass tubeplates of condensers with cast-iron water boxes. (d) Exposure of samples of tubing at selected stations in stagnant water under conditions promoting crevice corrosion and deposit attack. (e) Exposure of samples incorporating welded and expanded tube/tubeplate joints in the water boxes of operating condensers at selected power stations where tube trials were also in progress. (f) Trials at Brighton Sea-water Corrosion Laboratory to evaluate the performance of the tube materials themselves and of model tube/tubeplate assemblies containing both expanded and welded joints, in flowing, once-through sea-water. (g) Laboratory jet impingement tests. (h) Laboratory tests of plain tube and welded tube/tubeplate specimens to determine relative resistance of the various material categories to stress-corrosion and weld decay type corrosive environments. PROCEDURE AND RESULTS
Metallurgical examination of as-received tubing Lengths of the "as-received" tubing supplied initially were cut longitudinally on a diametral plane, to permit examination of the bore surface and, in particular, the welded seam. Metallographic examination of the tubes showed that the structure consisted of twinned equi-axed grains with traces of a fine carbide network at the grain boundaries
The evaluation of stainless steels
251
and within the grains. All the samples exhibited a fibrous structure after heavy etching presumably reflecting inhomogeneities in composition due to coring of the original ingot. In general, the grain size of the welded and annealed tubing was similar to that of the corresponding drawn material and both usually fell in the range AST M, Index 3-5 although both finer and coarser examples were observed. In the W A D A tubes the position of the weld was distinguishable as a band of more fine grained material. The structure within this band appeared to have been completely recrystallized although coring was still evident. All the tubes had hardnesses in the range 164-178 HV which suggested that the tubes had been slightly cold-worked as a result of post-annealing 'straightening and probably contained residual elastic stresses which might be relevant to their performance under corrosive conditions. Some tests were, therefore, carried out using the "Sachs" technique 9 to determine the residual hoop stress in samples of the tubes.
V
O
~7 • • /x • • o t~ O
Stella (North and South) P.S. Ferrybridge P.S. Fleetwood P.S. South Denes P.S. Tilbury P.S. Northfleet P.S. BlackwallPoint P.S. Uskrnouth P.S. Brighton P.S. FIG. 1.
252
D.W.C. BAKER,W. E. HEATONand B. C. PATIENT
The results indicated residual h o o p stress in the range 60--95 N / m 2 except 304L W A D A (I 14--145 N/ram 2) and 304L W A (105 N/mm2).
Field trials in operating condensers at power stations Scope of trials. Tubes o f 304, 304L, 316 and 316L material in the W A D A condition were installed as replacements in operating condensers at Northfleet, Tilbury, Blackwall Point, South Denes, Ferrybridge "B", Stella South, Stella N o r t h and Fleetwood Power Stations. In addition, tubes in the W A condition were installed at Northfleet. The trial at U s k m o u t h Power Station was confined to 316 W A D A tubing. The stations (Fig. 1) were selected because they covered a range o f cooling water conditions and included sites, such as Tilbury and Northtleet, which had a history o f condenser tube troubles. Two units were employed at Tilbury, one o f which was fitted with the Taprogge system of on-load cleaning ~° thereby allowing a direct assessment of the effect o f this process on the performance o f the tubes. The unit used at Northfleet was also fitted with the Taprogge system. Information on the water conditions obtaining at each station is given in Table 2. TABLE2. COOLH,,IOWATERCONDITIONSAT STATIONSWHERETRIALSWERECONDUCTED
Station Northfleet Tilbury Blackwall Point Ferrybridge 'B" Stella North Stella South
South Denes Fleetwood (4000 ppm Na2SO~) Uskmouth (suspended solids)2004600 ppm
Sodium chloride (ppm) Min Max 4100 20180 6000 21500 140 4130 37 236 1000 36500 (Average 17000) 20 22000 (Typical daily range: 20-4000). (Hourly variation: 8000-16000) 2570 18960
Conductivity (mho cm-~) Min 8750 11500 450 480 -.
4680
Average 38000
500
25500
Max 40000 40000 12000 1900 -.
Min 0"2 0"1 0"3 0.5 6.9 .
45000 68000
--
Dissolved oxygen (ppm)
--
pH
Max 3.9 8'0 5"5 9'0 8'6
7-I-7.7 7.0-7.3 7"0-7'3 6.9-7"1 7.8-8.3
5"6
8.9
7.5-8'2
--
5"0
8.4
--
6.0
7.7-8.0
.
.
Examination of tubes from condenser trials. N o t all the trials have been completed to date but all the tubes have been removed from the Tilbury No. 5, Northfleet and Blackwall Point condensers. The tubes installed in the Tilbury No. 5 condenser, which is fitted with Taprogge on-load cleaning, developed serious leaks after about 8 months' operation and were plugged. They were examined about 3 months later and it was found that tubes in all four material categories had suffered pitting and perforation. The damage increased in severity in the order: 316L, 316, 304L, 304. In each case the pits were r a n d o m l y
The evaluation of stainless steels
253
distributed and were not associated preferentially with the zone affected by welding. The history of the tubes in No. 1 condenser was very similar; serious leakage developed after about 5 months' operation and the tubes were plugged. They have not yet been recovered for examination. The specimens installed at Northfleet included tubes in both the WA and WADA condition. At the last site inspection, after nearly 2 y exposure, it was found that leakage had necessitated plugging of some of the tubes in each material category with the exception of the 316 WADA and 316L WADA tubes. Examination of samples of the tubes after their removal from the condenser confirmed that tubes in all the material categories had suffered severe pitting or perforation. Apart from pits, the tube bore surfaces, after removal of the deposit by scrubbing with water, appeared to be bright and indistinguishable from the new condition. The tubes installed at Blackwall Point were examined after 3 y exposure. A few very small pits were observed on the 316 WADA and 316L WADA tubes. The 304 WADA and 304L WADA tubes both exhibited a few rather larger pits, one or two of which penetrated more than one-half of the tube wall thickness. Some of the tubes installed at South Denes, Uskmouth and Stella South Power Stations have also been recovered for examination. A number of the 316 WADA tubes installed at South Denes Power Station were examined following the development of leaks. Again, the tubes exhibited pits and perforations in otherwise excellent material with a bright surface. A typical example is shown in Fig. 2. A 316 WADA tube removed at Uskmouth after leakage developed was found to exhibit pitting and perforation along the line of the welded zone. The remaining tubes were plugged as a precaution and have not yet been examined. The sample 316 tubes removed from Stella South Power Station showed a few superficial fine pits after 3 y expsure. Six tubes at Fleetwood Power Station were plugged in June 1969, following the development of leaks; the remainder continue in service. None has been removed for examination to date. No failures have been reported from Stella North, Stella South or Ferrybridge and the trials are continuing until it is convenient to remove the tubes for examination.
Behaviour of stainless steels in flowing once-through sea-water at Brighton Trials in experhnental tube rigs. In addition to the tests carried out by substituting experimental tubes in operating condensers, a series of tests were undertaken at the Brighton Sea-water Corrosion Laboratory, utilizing tube rigs through which sea-water could be passed on a "once-through" basis. Initially, it was intended to study the response of the different categories of tubing to various levels of cathodic protection and to the use of fibre-glass or cast-iron water boxes. The tubes were supported in inert "Alkathene" tubeplates and they were fitted with electrodes at various positions along their length to determine the potential distribution along the tube. The design of these is illustrated in Fig. 3 together with the electrical connections. The water velocity through the tubes was maintained constant at 5 ft s -1 (1.6 m s-l). Arrangements were made to monitor other parameters including temp., pH, 02 content, the conductivity of the sea-water, the tube electrode
254
D. W. C.
BAI~ER,W. E. HEATONand B. C. PATIENT Alkathene ~t r box
Anode
~Digital data recorder
Current measuring shunt Control panel
FIG.3. Electricalconnectionsto tuberigs. potentials and impressed cathodic currents. These were recorded on a data logging system and the results processed by computer, al Six rigs each capable of carrying twelve tubes were available, and a statisticallydesigned experiment was mounted to investigate the effects of the impressed current and water box material variables on the behaviour of the following categories of austenitic steel tubing: 304 304 304L 304L 316 316 316L 316L
WA WADA WA WADA WA WADA WA WADA
20 and 22 SWG ,, ,, ,, ,, ,, ,, ,,
In the event it was found that the tubes could not be polarized in the same manner as non-ferrous tubes and the detailed investigation of cathodic protection proved abortive. Furthermore, tube failures by perforation began to occur after only 2 weeks' operation and continued for the remaining 9 weeks of the test. Subsequent consideration of the results of the examination of the specimens from this test led to doubts about their validity, on the grounds that the performance of the tubes might have been affected by spurious electric currents flowing through the various connections to the tubes and the possibility that the physical presence of the electrodes might have
I')A n o . i t
(a)
Visual
appearance
FIG. 2.
FIO. 4.
r~. move. d
Appearance
(b) Section
through
pit
P i t in 3 1 6 L t u b e f r o m S o u t h D e n e s p o w e r s t a t i o n .
o f p i t in w e l d " ' b l i p " in 3 1 6 L W . T u b e t e s t e d a t B r i g h t o n S e a w a t e r C o r r o s i o n L a b o r a t o r y . x 5.
FIG. 6.
Showing appearance of (a) individual test plate before exposure, and (b) arrangement of plates in test rack.
FIG. 8.
Crevice and deposit corrosion test assembly after exposure at South Dcnes Power Station.
The evaluation of stainless steels
255
affected the water flow. For these reasons it was decided to conduct further tests without any electrical connection being made to any part of the rigs or the tubes, i.e. to use the nest merely as a convenient means of passing sea-water through the tubes. Since the initial trials had indicated that 304 and 304L materials we~:e inferior to 316 and 316L subsequent testing was concentrated on the latter materials. Several trials were carried out on tubes in the W, WA and W A D A conditions. The sea-water conditions during this period were within the following limits: pH 7.44-8.36, temp. 5.7-14.2°C, conductivity 25,650/48,050Mohm -1 and oxygen content 1.50-14.2 ppm. The behaviour of each category of material was inconsistent both within and between trials. However, the following general observations were made: (a) The tube bore surfaces were generally bright and "as-new". (b) Nearly all the tubes exhibited some fine pitting of the outside surface (which is exposed to the local marine atmosphere). (c) In some tubes, the welded bead over a distance of several inches was not properly formed, and the surface in the vicinity was discoloured, suggesting that the protective gas shield had failed during welding. (d) There were tubes in each category which exhibited pitting, some at welds and some in the body of the tube. A pit formed in the "blip" in the otherwise smooth profile of a weld in a 316L tube in the as-welded condition is shown in Fig. 4. Thus, while the level of pitting attack was less than observed during the first Brighton test, the results suggested that failure by this mechanism was a distinct possibility whether the material was of the "normal" or "low" C variety and whatever the condition of treatment. It also illustrated the danger of testing small specimens since the average number of pits detected was less than 1 every 20 ft of tubing. Jet impingement tests. The jet impingement rigs at the Brighton Laboratory are similar to those at the B N F M R A a2 except that they are fed with sea-water on a "oncethrough" basis. Water flow and air injection are controlled so that the jet impinges on the specimen (from a 2 m m nozzle positioned 2 m m from the surface) at a velocity of 4-6 m s -~ (15 ft s -x) with 3% air added in the form of large bubbles. Each rig accommodates twelve specimens which are in the form of half-section of a standard 1 in. o.d. condenser tube, 3 in. long. The normal period of test is 28 d; the specimens are rotated clockwise to adjacent specimen holders every working day to compensate for minor variations between jets. At the end of the test the specimens are washed, dried, weighed, and the dimensions of the pit determined. Type 316 WA and 316L WA specimens were prepared and tested so that the impingement region included the welded seams. After completing a standard test there was no sign of any damage due to impingement attack on any specimens. The only effect was a small rust-coloured area at the point of impingement; this was found to be fairly loose and only partially adherent and is presumed to have been deposited on the specimen surface from the sea-water which had picked up traces of Fe. There were no signs of crevice attack in the area of fixture to the specimen holder and the specimens showed no loss of weight. These observations suggested that the stainless steels offer very good resistance to inlet end impingement damage.
256
D.W.C.
BAKER,
W. E. HEATONand B. C. PATIENT
4~8 in
.
.
.
~ - ~ 8 Tubes on
-~
.
~ ]
l~6in,pitch(20
"
~-
SWG)
(i)
WELDED:
T u b e P l a t e and T u b e s of S a m e S p e c i f i c a t i o n . 1 , 3 , 5 and 7 We[ded A n n e a l e d D r a w n and A n n e a l e d . 2, 4, 6 and 8 W'elded and A n n e a l e d , Z, 4, 5 and 7 W e l d e d on Both F a c e s .
(u)
EXPANDED:
A l l T u b e s F i t t e d by E x p a n s i o n . (a) 316 T u b e P l a t e Tubes 2, 3, 5 and 8 Type 316 Tubes 1,4, 6 and 7 Type 316L (b) 304 T u b e P l a t e Tubes Z, 3, 5 and 8 Tubes I, 4, 6 and 7
Fit]. 5.
T y p e 304 Type 304L
Tube/tube plate specimens for exposure in water boxes of operating condensers.
Evaluation of welded and expanded tube/tubeplate assemblies Tests in water boxes of operating condensers. In order to assess the behaviour of tube/tubeplate joints in stainless steel, experimental tubeplate assemblies were prepared and installed in the water boxes of condensers at the same station as used • for the substitute condenser tube trials. In the case of the austenitic steel welded tube/ tubeplate assemblies the plate and tubes were nominally of matching specification. Welding was carried out using a "Tubesealer" automatic argon arc welding equipment (British Oxygen Co. Ltd.) with H F arc initiation. Each plate contained eight tubes, four in the welded and annealed condition and four in the welded, annealed, drawn and annealed condition. Two tubes of each category (i.e. four in all) were welded on one face of the tubeplate only while the remaining tubes were cut to a length to match the thickness of the tubeplate and welded on both faces. The latter technique was designed to introduce residual longitudinal contraction stresses which might influence behaviour in service. The arrangement of the tubes in each assembly is shown in Fig. 5.
The evaluation of stainless steels
257
Similar austenitic steel test plates were made containing expanded tubes but in this case both "normal" and "low" C varieties of the steel tubes were fitted in either a "normal" or "low" C tubeplate because of the limited availability ot~ these plate materials. Test plates of each type, i.e. as listed below, were assembled in mild steel racks by means of insulated tie rods in the order 316L welded, 316 welded, 316/316L expanded, 304/304L welded and expanded, 304L welded, and 304 welded. The rack and exposed mild steel parts were painted with an anti-corrosion paint and then fitted by a suitable bracket to a convenient structural member in the water box of a condenser at each of the seven test sites. The appearance of a test plate before test and of an assembly after testing are shown in Fig. 6. After exposure the test plates were visually and destructively examined. The expanded tubes were sawn along their length and sprung out of their holes so that the interface could be examined for crevice type corrosion both at the front face of the plate where a "tight" joint should have been obtained and at the back where the expansion terminated within the thickness of the tubeplate forming a crevice. It is impracticable to record the detailed results of the examination of every feature of every weld in the present paper. However, the broad conclusions were that crevice corrosion occurred both under the tie-rod washers and at the interface between the expanded tubes and tubeplates in all materials at one or more of the test sites. P!tting corrosion of both the plates and the tubes also occurred in all the material categories. In some instances pitting tended to occur preferentially either in the heat affected zone of the weld or on the weld bead itself. Tests at Brighton Sea-water Corrosion Laboratory. In addition to the tests carried out in power station condensers other test plates were exposed to flowing sea-water in a facility designed for that purpose at Brighton Sea-water Corrosion Laboratory. 'The form of the test plates used is illustrated in Fig. 7. The tubes were fitted either by welding or by expanding, using a controlled torque tool. The amount of expansion was varied within the tubes of each plate to determine whether there was a level of expansion below which there was a danger of crevice corrosion at the interface. Apart from the expanded 304 and 304L tubes which were fitted in the same 304 plates, the tubes and plates were of the same nominal specification. Up to nine plates were connected in series using standard plastics flanges (to B.S. 10, 1962, Table " E " ) and 5-in. bore plastic piping (Durapipe). Neoprene rubber gaskets were fitted between the tubeplate surfaces and the flanges to make a watertight sea. "['he plates were spaced about 36 in. apart in order to minimize the disturbanee in flow. The circulating water was taken from the inlet culverts of Brighton " A " Power Station at a flow of 3500 gal/h (6.3 1 s -1) corresponding to a velocity of about 1.3 m s -1 (4 ft s -1) through the tubes. Again it is impracticable to present the detailed results but the following general observations may be made: (1) In all the tests the plates became heavily encrusted with marine growths and deposits, including barnacles and worms. Corrosion of the tubeplate surfaces occurred under some of the marine growths. (2) A ring of crevice corrosion occurred around the periphery of the exposed part of the plate where the gasket made contact. After testing the plates were scrubbed
258
D. W. C. BAKER,W. E. HEATONand B. C. PATIENT 1420 S.W.G.TUBES.
•
9"
•
L
TUBES:
1, 3, S a n d 7
Welded Annealed
TUBES:
2, 4 a n d 6
Welded and Annealed.
Drawn and Annealed•
For Welded Assemblies: TUBES:
I, a n d 2
TUBES:
3, 4, 6, a n d 7 W e l d e d
Welded
on o n e face. o n both faces.
I~G. 7. Form of test plate used for evaluation of tube/tubeplate joints at Brighton Sea-water Corrosion Laboratory. down and examined visually with the aid of a low power microscope. Sections were prepared from selected areas for metallographic examination. All the materials suffered pitting corrosion to a greater or lesser extent. In general, the expanded joints showed crevice corrosion of the interfaces. There was no evidence that the amount of expansion, which varied from 3 to 24% wall thinning, had affected the extent of crevice corrosion.
Exposure of specimens under deposit and crevice corrosion conditions In the initial series of tests, samples of each tubing category were mounted in an epoxy resin base together with a brass supporting rod as shown in Fig. 8. The mount was formed by casting the resin around the base of the tubes located vertically in a suitable circular mould. The specimens in each assembly comprised lengths of 20 SWG tubing in the following material categories:
The evaluation of stainless steels 304 WADA 304L WADA
316 WADA 316L WADA
304 WA 304L WA
259 316 WA 316L WA
Rubber bands were placed around each tube about 1 in. from the top of the tube to form a crevice-like interface. In later repeat tests stainless steel bands made from tubes of matching composition were used instead of rubber. Assemblies of this type were immersed under relatively stagnant conditions such as reserve cooling water tanks or circulating water culverts at eight of the stations where condenser trials were carried out, i.e. at Northfleet, Blackwall Point, Tilbury, South Denes, Stella South, Stella North, Ferrybridge " B " and Fleetwood. These tests again demonstrated that all varieties of stainless steel tested were susceptible to crevice attack both at matching metal/metal junctions and at metal/rubber junctions. In many instances, pitting corrosion was observed. Damage was most severe in the test plates exposed at Tilbury, South Denes, and Fleetwood. In one test at Fleetwood Power Station, one 316 WADA tube had suffered gross general corrosion and "chemical polishing" of the bore where it had been filled with deposit. Laboratory stress-corrosion and weld decay tests Accelerated laboratory stress-corrosion and weld decay tests were carried out to determine whether there was any difference in behaviour between the various categories of steel and whether the tube/tubeplate welds were likely to be susceptible to failure by either of these mechanisms. The test specimens used for both types of test were similar and comprised: (a) Lengths of plain tubing as received and after annealing at 1000°/1050°C. (b) Single tube/tubeplate welded specimens. (c) Tube/tubeplate specimens welded on both faces of the plate to introduce additional residual stresses. The plain tube specimens were prepared by carefully sawing through the tubing while it was clamped firmly in a jig designed to prevent distortion during cutting. It was found, during preliminary commissioning tests of the apparatus, that many fine cracks were generated at the sawn ends of the tubing and they were subsequently dressed by filing and grinding (to grade 600 paper) to obviate this preferential attack. Stress-corrosion tests. The initial stress-corrosion tests were carried out in boiling 42%MgC12. lz The specimen was removed from the apparatus every 24 h, washed in water, dried and examined with the aid of a hand-lens. The time taken for cracks to appear was noted. The tests were terminated after a total of 96 h exposure and sections taken for metallographic examination. Subsequently some tests were carried out at room temp. on welded specimens in a solution of 3 %NaC1 saturated with H~S. 14 (a) Plain tube tests. Although the behaviour of the plain tube specimens in the tests carried out in boiling 42%MGC12 solution was variable, it was found that in the "as received" condition all four varieties (316, 316L, 304 and 304L) were susceptible to cracking. Except in the case of the 304L WA tubing, which appeared to be particularly susceptible, annealing eliminated cracking although pitting attack occurred in most instances. The latter may be attributable to the superficial oxidation which occurred during the prior heat treatment of the specimens and impaired the general corrosion resistance of the material.
260
D.W.C. BAKER,W. E. HEATONand B. C. PATIENT
There was no evidence that the presence of the welded seam in the WA or W A D A specimens was preferentially susceptible to cracking. The cracks which did occur tended to lie parallel to the axis of the tubing and in some instance they extended the whole length of the specimen, the tube springing open confirming the presence of residual stresses in the hoop direction in the as-received tubing.
(b) Tube/tubeplate specimen tests. Tests were carried out in boiling MgC12 solution on the tube/tubeplate specimens in the same manner as those on the plain tube specimens. Again the response of the specimens differed in replicate tests in many instances. However, it was clear that all the material categories were susceptible. The incidence of cracking of the tubeplate tended to be greater in the immediate vicinity of the weld bead. Cracking of the tubes was also concentrated in many instances in the heat affected zone about 1-2 mm from the edge of the weld bead in the area where a circumferential band in the bore of the tube was discoloured by oxidation during welding. In a number of instances the cracking of this zone was such that the tube fractured and broke away during preparation of sections. There was no evidence that the specimens welded on both faces behaved differently from those welded on only one face. Metallographic examination of sections of the tube/tubeplate specimens showed that the cracking was transgranular in all cases, and did not appear to deviate on passing from one structural zone to another, e.g. from the tubeplate into the weld metal. In addition to the tests carried out in boiling MgCI 2 solution tests were also made in a 3%NaCI solution saturated with H2S at room temp. This solution was used by Phelps and Mears 14 in the course of a study of the relationship between composition and structure of stainless steels and their resistance to stress--corrosion. It was chosen since it approximated to that which might occur in practice at a polluted estuarine site and it was reported to have caused cracking of Type 304 steel in the cold-worked condition at room temp. Tests were carried out on 316 WA, 316 WADA, 316L WA, 316L WADA, 304 WA, 304 WADA, 304L WA, and 304L WADA, tube/tubeplate specimens welded on one and both faces respectively. The solution was used at room temp. and the concentration of sulphide was maintained by bubbling H2S gas through the solution for two 10-min periods each day. The tests were continued for 96 h and the specimen examined microscopically before and after sectioning. The only specimens which showed signs of significant deterioration were both types of welded 316 specimens with W A D A tubes. In these specimens cracks were observed both in the tubeplate and in the weld metal. The cracking was shallow and metallographic examination suggested that it was initiated at the surface of the tubeplate which contained evidence of cold work as a result of machining. Welddecay tests. The weld decay tests were carried out in a boiling H2SOJCuSO4.14 The specimens were cut from the tubes in the same manner as the stress-corrosion specimens and weighed. The apparatus was as used for the stress-corrosion tests, the flask containing 600 ml of the test solution. The solutions were brought to the boil and allowed to reflux gently for 72 h after which the specimens were removed, washed, dried and reweighed.
The evaluation of stainless steels
261
At least two tests were carried out on each type of tubing. In addition to weight-loss measurements the specimen were examined microscopically and mechanically tested by flattening and expansion tests. In the flattening tests the tube was flattened by hammer blows until the minimum dimension was reduced to one-half the original dia. In the expansion test a taper drift wiih an included angle of 45°was driven into the end of the tube until the dia. at the mouth was greater than the original dia. by at least 50°;. All of the samples withstood these tests without cracking. Some fine pitting occurred in all the specimens but it tended to be more marked in the 316 and 316L tubing than the 304 and 304L and greater in the welded and annealed tubing than in the welded, annealed, drawn and annealed tubing. Tests were also carried out on welded tube/tubeplate specimens both on those welded on one face only and those welded on both faces. No significant weight-loss measurements could be made because of disproportionate mass of the tubeplate compared with that of the tubing. None of the samples showed evidence of deterioration.
DISCUSSION The complete results of the field trials are not yet available but it is clear that all the materials (i.e. 316L, 316, 304L and 304) even in the preferred condition, i.e. welded, annealed, drawn and annealed, are susceptible to corrosion. This has ranged from superficial pitting exhibited by the tubes exposed, for example, at Bl'ackwall Point, to severe perforation and pitting suffered by the tubes at the estuarine sites at Tilbury, Northfleet, South Denes, Fleetwood and Uskmouth. It is also evident from the tests at the Brighton Sea-water Corrosion Laboratory that these materials are not suitable for service in sea-water cooled condensers. The Mo bearing steels (316 and 316L) have in general behaved better than the straight Cr-Ni steels but the difference, if any, has been one of degree rather than kind. One curious feature of the Tilbury tube trials was that in both tests all four different categories of tubing failed simultaneously after the same period of exposure. The fact that the tubing survived longer at Fleetwood and the Stella stations than at Tilbury, although the CI concentrations in the cooling waters are similar, may indicate that the levels of pollution and residual 02 are important in maintaining the integrity of the surface film on austenitic steels. At Northfleet and Brighton, tubes in the welded and annealed condition were also tested. In general, there is no evidence that the performance of the weld was markedly different from that of the parent material in the sense that failure occurred both at the welded seam and elsewhere. However, there were some pits that were located so precisely on the weld seam that it seems likely they were initiated by some local feature. The stainless steel tubes in the site trials, mounted in conventional condensers, would be expected to be slightly cathodic with respect to both the brass tubeplate and cast-iron water box. There was no evidence that this electrochemical effect had had any influence on the behaviour of the tubes or the tubeplates in the immediate vicinity of the stainless steel sample tubes.
262
D.W.C. BAKER,W. E. HEATONand B. C. PATIENT
An aspect of the tests carried out at Brighton Sea-water Corrosion Laboratory was the variability of behaviour of the materials. Exhaustive attempts were made to determine the reasons for the variable behaviour at Brighton by detailed chemical analyses, metallographic examination and hardness testing of the different materials. No correlation between the incidence of pitting corrosion and any structural or compositional feature could be found. The tube rig tests at Brighton, the tests of welded and expanded tubeplate assemblies and the test on tubes under relatively stagnant water conditions have all shown that the austenitic stainless steels are very sensitive to crevice corrosion both at interfaces between metal parts of matching composition and at interfaces between the steels and non-metallic materials, e.g. rubber and marine organisms. It was not possible to define any geometrical limits which offered immunity. The catastrophic damage suffered underneath neoprene gaskets in the tests in sea-water certainly show that once this mode of attack has started the clearance does not appear to be critical and can certainly be greater than 1 mm. At the other end of the scale the negligible clearance offered by a mechanically expanded joint did not prevent attack although it was less marked than in sliding fits. The form of pitting observed in the Brighton rigs (which carried no heat load) was so similar to the pitting observed from tubes in operational condensers that thermal effects were not considered to be playing any part in the corrosion processes. The jet impingement tests in flowing sea-water showed no susceptibility either of the 316 or 316L tube materials or welded seams to attack by impingement and this is supported by the absence of any impingement type damage in the condenser trials. In view of the performance of the tubing itself, the behaviour of welded joints is no longer crucial but the evidence suggests that welds in these materials are only marginally inferior to the parent materials in their resistance to pitting attack. The investigation has again emphasized the difficulty in carrying out meaningful corrosion tests on materials such as stainless steels which depend for their survival on the integrity of passive surface film. Many of the specimens examined in the present work have appeared, over the vast bulk of their surface, to be in as good condition as when they entered service but here and there serious pits and perforations have developed. Sometimes there has been some obvious correlation of the pit and a local feature, for example, an adherent marine organism, a discontinuity in the weld or a manufacturing defect in the tube; but in the majority of instances there is no residual evidence that the site of the pit was in any way different from the remainder of the surface. Furthermore, similar features and flaws have, in the majority of cases, not been the source of pitting. The laboratory stress-corrosion and weld decay tests showed little difference between the various material categories studied but there was some indication that the heat-affected zones of welds would prove the most sensitive to cracking if a stresscorrosion situation did arise in practice. In fact, cracking was not observed in any of the operational or rig trials. The reason why the performance of the stainless steels in these trials is inferior to that reported from the U.S.A. is uncertain but it may reflect a fundamental difference in the water conditions. Cleanliness of condenser tubes has always been emphasized in American publications on this subject and there have been rumours
The evaluation of stainless steels
263
o f serious failures by pitting a t t a c k in condensers which have been left filled with stagnant water. However, at Tilbury a test was carried out in one condenser o p e r a t i n g with T a p r o g g e o n - l o a d cleaning a n d the tubes p e r f o r m e d no better t h a n those in a neighbouring unit o p e r a t i n g without such facilities. There is no evidence t h a t these tubes were allowed to stand in stagnant water for any significant lengtla o f time a n d yet they failed in a few months. CONCLUSIONS (1) The austenitic stainless steel tubes p r o d u c e d by a T I G welding process in A1SI Types 316, 316L, 304 and 304L steels are unsuitable for c o n d e n s e r tube service in sea-water a n d polluted estuarine environments. (2) The austenitic steels are particularly susceptible to crevice c o r r o s i o n at interfaces such as r u b b e r - m e t a l joints and t u b e / t u b e p l a t e expansions in the cooling water environments studied. (3) A p a r t from crevice corrosion, the chief m o d e o f failure is by pitting c o r r o s i o n . The M o bearing steels (316 and 316L) are distinctly better than the straight C r - N i steels (304 a n d 304L) in this respect but are not immune. (4) The p e r f o r m a n c e o f weld in the austenitic stainless steels was n o t significantly different from parent tube or tubeplate material although there were situations where corrosion has initiated at a local feature o f a weld. (5) The Type 316 a n d 316L austenitic steels are resistant to i m p i n g e m e n t a t t a c k under the condition o f the jet i m p i n g e m e n t test using flowing sea-water and are superior to conventional non-ferrous condenser tube alloys in this respect. Acknowledgements--The authors would like to express their thanks to the many people who assisted
in the investigation: to the Station Superintendents at Northfleet, Tilbury, Blackwall Point, South Denes, Flcetwood, Ferrybridge, Stella South, Stella North and Uskmouth Power Stations and their respective staffs, particularly the Station Chemists; to their colleages in North Western, North Eastern and South Western Regions' Scientific Service Departments for assistance with the preparation of test-pieces and their exposure at the test sites. They would also like to acknowledge the contribution of the late Mr. W. Matthewman to the work at the Brighton Sea-water Corrosion Laboratory and to their colleagues at Cockfosters, particularly Mr. M. F. Jones and Mr. P. Mukerjee, who assisted with the examination of the test specimens. The paper is published by permission of Mr. R. A. Peddie, Director General, South Eastern Region, Central Electricity Generating Board. REFERENCES 1. H. J. VYHMALEK,27th Meeting of the American Power Conference, Chicago, Illinois (1965). 2. J. H. HARLOW,ASME-IEE Power Conference, San Francisco (1961). 3. J. R. MAURER, Basic Considerations in the Use of Stainless Steels in Modern Condenser Design. The International Nickel Power Conference, Estes Park, Colorado (August 1961). 4. N. A. LONG,Recent Operating Experiences with Stainless Steel Condenser Tubes. American Power Conference, Chicago (1966). 5. F. L. LAQOEand M. A. CORDOVl,Experiences with Stainless SteeI Surface Condemers in the U.S.A. Paper No. 12, International Nickel Power Conference, Lausanne (1967). 6. V. N. GULYAEVand M. I. LUZHNOV, 'The Choice of Materials for Condenser Tubes', Thermal Eng. 11 (3), 85 (1964). 7. R. GASPARINIand P. STURLA,Experience and Selection of ColMenser Materials for Power Plants. Paper No. 10, INCO Power Conference, Lausanne (1967). 8. British Standard No. 378. 9. G. SACHSand VAN HORN,Practical Metallurgy, p. 181. ASM, Cleveland, Ohio (1951). 10. Anon, Engineering, June 27, 1958, British Patent 700, 833.
264
D . W . C . BAKER,W. E. HEATON and B. C. PATIENT
W. MATTHEWMANand W. E. HEATON,Corros. Sci. 8, 453 (1968). R.. MAY and R.. W. DE VERE STACPOOLE,J. Inst. Metals 77, 351 (1958). M. A. SCHEIL et al., Weldingd. 22, 493/5 (1953). E. H. PHELPS and R. B. MEARS, The Effect o f Composition and Structure o f Stainless Steels upon Resistance to Stress Corrosion CracKing, I st International Congress on Metallic Corrosion, p. 319. Butterworths, London (1962). 15. J. H. G. MONEYPENNY, Stainless Iron andSteel, Vol. 1 (3rd Ed.), p. 99. Chapman & Hall, London (1951). 11. 12. 13. 14.
THE EVALUATION OF STAINLESS STEELS FOR CONDENSER TUBE NEST CONSTRUCTION* D. W. C. BAKER, W. E. HEATON~" a n d B. C. PATIENT Central Electricity Generating Board, Scientific Services Department, West Farm Place, Chalk Lane, Cockfosters, Barnet, Herts, England Abstract--The possibility of using austenitic stainless steel for condenser tube nest construction for CEGB plant was investigated by laboratory and field trials. It was found that both the Mo bearing (AISI Type 316 and 316L) steels and the straight 18% Cr-10%Ni (AISI Type 304 and 304L) steels suffered crevice and pitting corrosion on exposure to sea and estuarine cooling waters and cannot be recommended for service in the U.K. under these conditions. R6sum6---Ona 6tudi6 en laboratoire et en usine la possibilit6 d'utiliser des aciers inoxydables aust6nitiques pour la confection de tubes de condenseurs pour l'usine de la CEBG. On a trouv6 que les aciers des types AISI 316 et 316 L au molybd6ne et les aciers de type AISI 304 et 304 L it 18% de chrome et 10% de nickel subissent tous deux une corrosion caverneuse et par piqfires en cas d'exposition gt des eaux de refroidissement marine et d'estuaire, et que leur utilisation ne peut pas 6tre recommand6e dans le Royaume Uni dans de telles conditions. Zusammenfassung--Die M6glichkeit, austenitischen rostfreien Stahl fiir die Kondensatorrol~rbiJndelKonstruktion einer CEGB-Anlage zu verwenden, wurde im Laboratorium und praktisch untersucht. Es wurde festgestellt, dass die Mo-haltigen St~hle (AISI Type 316 und 316L) und die St~ihle mit 18%Cr und I0%Ni (AISI Type 304 und 304L) durch Einwirkung von Ktihlwasser aus dem Meer und aus FlussmiJndungen Spalten- und Narbenkorrosion erlitten und zur Verwendung in Grossbritannien unter solchen Bedingungen nicht empfohlen werden kfnnen. INTR.ODUCTION THE POSSIBILITY of using stainless steels for the c o n s t r u c t i o n of t u r b i n e condenser tube nests for C E G B plant was considered for the following reasons: (a) T o eliminate a potential source of c o n t a m i n a t i o n of the condensate a n d hence the feedwater by C u a n d other metals leached from c o n v e n t i o n a l non-ferrous tube alloys. (b) T o improve the integrity of condensers by permitting welding o f tube/tubeplate joints. (c) To replace the established tube alloys where technical or economic advantages could be shown. The problems arising from C u c o n t a m i n a t i o n of the water entering the boiler, including the deposition of C u oxides in the turbine, are intensified in " o n c e - t h r o u g h " supercritical pressure systems. 1,2 At the time of starting the present investigation (1965), it was proposed that the 660 m W units to be installed at Drax (Yorkshire) would operate at supercritical pressure and there was an immediate need to eliminate C u bearing materials from the water circuit. It was not k n o w n whether the c o n d e n s e r *Manuscript received 11 May 1971 ; in revised form 24 June 1971. tCentral Electricity Generating Board, Scientific Services Department, Sea-water Corrosion Laboratory, Brighton Power Station, Portslade, Brighton, Sussex, BN4 lWG. 247
D. W. C.
248 TABLE 1.
BAKER,
W. E. HEATONand B. C. PATIENT
VARIATION OF FEED AND BOILER DRUM WATER SPECIFICATIONS WITH UNIT SIZE AND STEAM CONDITIONS
Steam conditions
6"25 N/mm z, 485°C 900 lb/in. °(905°F)
10-4 N/mm -~, 565°C "1500 lb/in.'-" (1050°F)
Commissioning date and size
1952 60 mW
1957 120 mW
16.2 N/ram ~-,565'~C 2350 Ib/in. 2 (1050°F) 1963 200 mW
1967 500 mW
Feed water
Conductivity-micromho/cm Dissolved oxygen --ppm Metals--ppm pH value
~ 1.6
:b 0"5 ~r 0'3 (after cation exchange to hydrogen chloride)
> 0.02 Not specified
:b 0.007 (Fe + Cu):l- 0'01
:b 0.007 (Fe-I- Cu):b 0.01
8.0-8.5
Not specified directly
8.8-9.2 at 25°C
:b 33 as NaC1 ::1- 3.0 as SiO~
21" 16 as NaCI 21- 0-3 as SiO~
:~ 0'3 21, 0.007 (Fe+Cu +Ni):b 0.01 8'5 at 25°C
Boiler drum water
Chloride--ppm Silica--ppm
> 50 as NaCI :b 6-0 as SiO.~
> 4 as NaCI :b 0.2 as SiO2
would be, in fact, a significant source o f Cu pick-up and work was also initiated to cover this aspect. In the event it was decided to install subcritical pressure drum units at Drax and the need for alternative condenser materials diminished. However, the prospect o f future advanced cycle A G R stations, located on the coast, maintained interest in the future application of stainless steels in a marine environment. The increasing stringency of feedwater purity requirements (Table 1) for advanced units also emphasized the need for improved integrity o f condensers. Mechanical methods o f tube fixing using expansion or packing do not eliminate potential interface leak paths, and welding appears to offer an alternative to the use of double tubeplates, which present their own problems and economic penalties. The belief that austenitic stainless steels would assist in this respect was encouraged by the knowledge that they offer much better welding characteristics than the non-ferrous alloys. The continuing corrosion and erosion problems experienced with non-ferrous alloys in aggressive sea-water and estuarine environments also warranted the investigation of alternative materials. Stainless steels were used initially in the U.S.A. for condenser tubing to overcome specific problems in the steam impingement and incondensible gas exhaust zones o f condensers. 3 Successful experience led to their wider use as replacements for established non-ferrous alloys where cooling water corrosion problems were met. 4 It has been stated that the majority o f new condensers in the U.S.A. are now partially or entirely tubed with austenitic stainless steel. 5 However, the cost o f austenitic steel tubing relative to that o f brass in the U.S.A. may be the dominant factor. Interest in the possibility of using stainless steels to replace Cu alloys has also been stimulated in Eastern 6 and Western Europe. 7 The work reported here was part o f a collaborative programme o f evaluation carried out with major U.K. turbine-makers, namely: English Electric Co. Ltd., Associated Electrical Industries Ltd., and C. A. Parsons Ltd.
The evaluation of stainless steels
249
MATERIALS INVESTIGATED The cost of austenitic steel tubes made by tungsten electrode inert gas (TIG) welding of preformed rolled strip is significantly less than that of seamless tubing produced by drawing from forged or extruded hollows. For this reason, the evaluation was carried out on welded tubing produced in this manner. While tubing can be produced to the required dimensional tolerances s in the "as welded" condition it was recommended by the manufacturers, following American practice, that after welding the tubing should be annealed and then cold-drawn and reannealed to ensure that the weld zone is completely recrystallized. Each of the additional processes adds significantly to the final cost. In the present investigation "as welded", "welded and annealed" and "welded, annealed, drawn and reannealed" tubing was examined because, apart from the economic advantages of minimizing post-welding treatments, the behaviour of the seam welds would give guidance on the likely performance of welded tube/tubeplate joints which it would not be practicable to work or heat-treat after welding. Four different austenitic stainless steels were selected for investigation, covering the "normal" and "low" C varieties of both the straight Cr-Ni and the Mo bearing alloys: °'oCr
%Ni
?'oMo
%C
Type 304
18/20
8/12
--
0.08 maximum
Type 304L
18/20
8/12
--
0.03
,,
Type 316
16/18
10/14
2/3
0-08
,,
Type 316L
16/18
10/14
2/3
0-03
,,
Tile plain Cr-Ni steels (Type 304 and 304L) are claimed to be satisfactory in nonaggressive waters but the more expensive Mo steels (Type 316 and 316L) are required for brackish or sea-waters. Steels of the "low" C type were included because they offer greater resistance to intercrystalline corrosion of the heat affected zones of welds. They are, however, more costly than the corresponding "normal" C varieties. Apart from the obvious economic implications, the investigation of a range of steels of differentcomposition and treatment offered the chance, if they showed different responses, of characterizing the various test environments and defining the limits of suitability of each category. To conform with existing practice, 25.4 mm o.dia, tubing was used for the investigation, but the high strength and modulus of the steels compared with nonferrous tube alloys allowed a reduction in thickness of the tubing compared with the non-ferrous materials to be considered. For field trials in operating condensers 18 SWG (0.048 in., 1"2 mm) tubing was used to match the other tubes in the nest but both 20 SWG (0.036 in., 0.9 ram) and 22 SWG (0.028 in., 0.7 mm) tubing was used for the other parts of the programme, including the welding trials, since they appeared to be feasible for condenser construction from a mechanical standpoint. To summarize, the various categories of steel tubing which were evaluated are listed below; in all cases the analyses conformed to specification.
250
D.W.C. BAKER,W. E. HEATONand B. C. PATIENT
Austenitic Type 304 ,, ,, ,, ,,
Type 316 Type 316L Type 304 Type 304L
,, ,, ,, ,, ,, ,,
Type 316 Type 316L Type 304 Type 304L Type 316 Type 316L
as welded (subsequently referred to as 304 W) (typical composition 0.06%C, 18.2%Cr, 9.5%Ni) as welded (316 W) (0.05%C, 17.4%Cr, 11.4%Ni, 2.5%Mo) as welded (316L W) (0.02%C, 17.4%Cr, 12.6%Ni, 2.5%Mo) welded and'annealed (304 WA) welded and annealed (304LWA) (0.03%C, 17.3%Cr, 12%Ni) welded and annealed (316 WA) welded and annealed (316L WA) welded, annealed, drawn and annealed (304 WADA) welded, annealed, drawn and annealed (304L WADA) welded, annealed, drawn and annealed (316 WADA) welded, annealed, drawn and annealed (316L WADA).
For those parts of the programme requiring tubeplate materials, e.g. the welding trials, plate or bar material of matching specification was used as far as possible. EVALUATION PROGRAMME The complete programme covered the following aspects: (a) Metallurgical examination of tubing as supplied. (b) Development of tube/tubeplate welding techniques. This work was undertaken by North Western Region Scientific Services Department, CEGB, at their Didsbury Laboratories. (c) Replacement of non-ferrous tubing in operating condensers using various types of cooling water. The stainless steel tubes were fixed by conventional means into the existing naval brass tubeplates of condensers with cast-iron water boxes. (d) Exposure of samples of tubing at selected stations in stagnant water under conditions promoting crevice corrosion and deposit attack. (e) Exposure of samples incorporating welded and expanded tube/tubeplate joints in the water boxes of operating condensers at selected power stations where tube trials were also in progress. (f) Trials at Brighton Sea-water Corrosion Laboratory to evaluate the performance of the tube materials themselves and of model tube/tubeplate assemblies containing both expanded and welded joints, in flowing, once-through sea-water. (g) Laboratory jet impingement tests. (h) Laboratory tests of plain tube and welded tube/tubeplate specimens to determine relative resistance of the various material categories to stress-corrosion and weld decay type corrosive environments. PROCEDURE AND RESULTS
Metallurgical examination of as-received tubing Lengths of the "as-received" tubing supplied initially were cut longitudinally on a diametral plane, to permit examination of the bore surface and, in particular, the welded seam. Metallographic examination of the tubes showed that the structure consisted of twinned equi-axed grains with traces of a fine carbide network at the grain boundaries
The evaluation of stainless steels
251
and within the grains. All the samples exhibited a fibrous structure after heavy etching presumably reflecting inhomogeneities in composition due to coring of the original ingot. In general, the grain size of the welded and annealed tubing was similar to that of the corresponding drawn material and both usually fell in the range AST M, Index 3-5 although both finer and coarser examples were observed. In the W A D A tubes the position of the weld was distinguishable as a band of more fine grained material. The structure within this band appeared to have been completely recrystallized although coring was still evident. All the tubes had hardnesses in the range 164-178 HV which suggested that the tubes had been slightly cold-worked as a result of post-annealing 'straightening and probably contained residual elastic stresses which might be relevant to their performance under corrosive conditions. Some tests were, therefore, carried out using the "Sachs" technique 9 to determine the residual hoop stress in samples of the tubes.
V
O
~7 • • /x • • o t~ O
Stella (North and South) P.S. Ferrybridge P.S. Fleetwood P.S. South Denes P.S. Tilbury P.S. Northfleet P.S. BlackwallPoint P.S. Uskrnouth P.S. Brighton P.S. FIG. 1.
252
D.W.C. BAKER,W. E. HEATONand B. C. PATIENT
The results indicated residual h o o p stress in the range 60--95 N / m 2 except 304L W A D A (I 14--145 N/ram 2) and 304L W A (105 N/mm2).
Field trials in operating condensers at power stations Scope of trials. Tubes o f 304, 304L, 316 and 316L material in the W A D A condition were installed as replacements in operating condensers at Northfleet, Tilbury, Blackwall Point, South Denes, Ferrybridge "B", Stella South, Stella N o r t h and Fleetwood Power Stations. In addition, tubes in the W A condition were installed at Northfleet. The trial at U s k m o u t h Power Station was confined to 316 W A D A tubing. The stations (Fig. 1) were selected because they covered a range o f cooling water conditions and included sites, such as Tilbury and Northtleet, which had a history o f condenser tube troubles. Two units were employed at Tilbury, one o f which was fitted with the Taprogge system of on-load cleaning ~° thereby allowing a direct assessment of the effect o f this process on the performance o f the tubes. The unit used at Northfleet was also fitted with the Taprogge system. Information on the water conditions obtaining at each station is given in Table 2. TABLE2. COOLH,,IOWATERCONDITIONSAT STATIONSWHERETRIALSWERECONDUCTED
Station Northfleet Tilbury Blackwall Point Ferrybridge 'B" Stella North Stella South
South Denes Fleetwood (4000 ppm Na2SO~) Uskmouth (suspended solids)2004600 ppm
Sodium chloride (ppm) Min Max 4100 20180 6000 21500 140 4130 37 236 1000 36500 (Average 17000) 20 22000 (Typical daily range: 20-4000). (Hourly variation: 8000-16000) 2570 18960
Conductivity (mho cm-~) Min 8750 11500 450 480 -.
4680
Average 38000
500
25500
Max 40000 40000 12000 1900 -.
Min 0"2 0"1 0"3 0.5 6.9 .
45000 68000
--
Dissolved oxygen (ppm)
--
pH
Max 3.9 8'0 5"5 9'0 8'6
7-I-7.7 7.0-7.3 7"0-7'3 6.9-7"1 7.8-8.3
5"6
8.9
7.5-8'2
--
5"0
8.4
--
6.0
7.7-8.0
.
.
Examination of tubes from condenser trials. N o t all the trials have been completed to date but all the tubes have been removed from the Tilbury No. 5, Northfleet and Blackwall Point condensers. The tubes installed in the Tilbury No. 5 condenser, which is fitted with Taprogge on-load cleaning, developed serious leaks after about 8 months' operation and were plugged. They were examined about 3 months later and it was found that tubes in all four material categories had suffered pitting and perforation. The damage increased in severity in the order: 316L, 316, 304L, 304. In each case the pits were r a n d o m l y
The evaluation of stainless steels
253
distributed and were not associated preferentially with the zone affected by welding. The history of the tubes in No. 1 condenser was very similar; serious leakage developed after about 5 months' operation and the tubes were plugged. They have not yet been recovered for examination. The specimens installed at Northfleet included tubes in both the WA and WADA condition. At the last site inspection, after nearly 2 y exposure, it was found that leakage had necessitated plugging of some of the tubes in each material category with the exception of the 316 WADA and 316L WADA tubes. Examination of samples of the tubes after their removal from the condenser confirmed that tubes in all the material categories had suffered severe pitting or perforation. Apart from pits, the tube bore surfaces, after removal of the deposit by scrubbing with water, appeared to be bright and indistinguishable from the new condition. The tubes installed at Blackwall Point were examined after 3 y exposure. A few very small pits were observed on the 316 WADA and 316L WADA tubes. The 304 WADA and 304L WADA tubes both exhibited a few rather larger pits, one or two of which penetrated more than one-half of the tube wall thickness. Some of the tubes installed at South Denes, Uskmouth and Stella South Power Stations have also been recovered for examination. A number of the 316 WADA tubes installed at South Denes Power Station were examined following the development of leaks. Again, the tubes exhibited pits and perforations in otherwise excellent material with a bright surface. A typical example is shown in Fig. 2. A 316 WADA tube removed at Uskmouth after leakage developed was found to exhibit pitting and perforation along the line of the welded zone. The remaining tubes were plugged as a precaution and have not yet been examined. The sample 316 tubes removed from Stella South Power Station showed a few superficial fine pits after 3 y expsure. Six tubes at Fleetwood Power Station were plugged in June 1969, following the development of leaks; the remainder continue in service. None has been removed for examination to date. No failures have been reported from Stella North, Stella South or Ferrybridge and the trials are continuing until it is convenient to remove the tubes for examination.
Behaviour of stainless steels in flowing once-through sea-water at Brighton Trials in experhnental tube rigs. In addition to the tests carried out by substituting experimental tubes in operating condensers, a series of tests were undertaken at the Brighton Sea-water Corrosion Laboratory, utilizing tube rigs through which sea-water could be passed on a "once-through" basis. Initially, it was intended to study the response of the different categories of tubing to various levels of cathodic protection and to the use of fibre-glass or cast-iron water boxes. The tubes were supported in inert "Alkathene" tubeplates and they were fitted with electrodes at various positions along their length to determine the potential distribution along the tube. The design of these is illustrated in Fig. 3 together with the electrical connections. The water velocity through the tubes was maintained constant at 5 ft s -1 (1.6 m s-l). Arrangements were made to monitor other parameters including temp., pH, 02 content, the conductivity of the sea-water, the tube electrode
254
D. W. C.
BAI~ER,W. E. HEATONand B. C. PATIENT Alkathene ~t r box
Anode
~Digital data recorder
Current measuring shunt Control panel
FIG.3. Electricalconnectionsto tuberigs. potentials and impressed cathodic currents. These were recorded on a data logging system and the results processed by computer, al Six rigs each capable of carrying twelve tubes were available, and a statisticallydesigned experiment was mounted to investigate the effects of the impressed current and water box material variables on the behaviour of the following categories of austenitic steel tubing: 304 304 304L 304L 316 316 316L 316L
WA WADA WA WADA WA WADA WA WADA
20 and 22 SWG ,, ,, ,, ,, ,, ,, ,,
In the event it was found that the tubes could not be polarized in the same manner as non-ferrous tubes and the detailed investigation of cathodic protection proved abortive. Furthermore, tube failures by perforation began to occur after only 2 weeks' operation and continued for the remaining 9 weeks of the test. Subsequent consideration of the results of the examination of the specimens from this test led to doubts about their validity, on the grounds that the performance of the tubes might have been affected by spurious electric currents flowing through the various connections to the tubes and the possibility that the physical presence of the electrodes might have
I')A n o . i t
(a)
Visual
appearance
FIG. 2.
FIO. 4.
r~. move. d
Appearance
(b) Section
through
pit
P i t in 3 1 6 L t u b e f r o m S o u t h D e n e s p o w e r s t a t i o n .
o f p i t in w e l d " ' b l i p " in 3 1 6 L W . T u b e t e s t e d a t B r i g h t o n S e a w a t e r C o r r o s i o n L a b o r a t o r y . x 5.
FIG. 6.
Showing appearance of (a) individual test plate before exposure, and (b) arrangement of plates in test rack.
FIG. 8.
Crevice and deposit corrosion test assembly after exposure at South Dcnes Power Station.
The evaluation of stainless steels
255
affected the water flow. For these reasons it was decided to conduct further tests without any electrical connection being made to any part of the rigs or the tubes, i.e. to use the nest merely as a convenient means of passing sea-water through the tubes. Since the initial trials had indicated that 304 and 304L materials we~:e inferior to 316 and 316L subsequent testing was concentrated on the latter materials. Several trials were carried out on tubes in the W, WA and W A D A conditions. The sea-water conditions during this period were within the following limits: pH 7.44-8.36, temp. 5.7-14.2°C, conductivity 25,650/48,050Mohm -1 and oxygen content 1.50-14.2 ppm. The behaviour of each category of material was inconsistent both within and between trials. However, the following general observations were made: (a) The tube bore surfaces were generally bright and "as-new". (b) Nearly all the tubes exhibited some fine pitting of the outside surface (which is exposed to the local marine atmosphere). (c) In some tubes, the welded bead over a distance of several inches was not properly formed, and the surface in the vicinity was discoloured, suggesting that the protective gas shield had failed during welding. (d) There were tubes in each category which exhibited pitting, some at welds and some in the body of the tube. A pit formed in the "blip" in the otherwise smooth profile of a weld in a 316L tube in the as-welded condition is shown in Fig. 4. Thus, while the level of pitting attack was less than observed during the first Brighton test, the results suggested that failure by this mechanism was a distinct possibility whether the material was of the "normal" or "low" C variety and whatever the condition of treatment. It also illustrated the danger of testing small specimens since the average number of pits detected was less than 1 every 20 ft of tubing. Jet impingement tests. The jet impingement rigs at the Brighton Laboratory are similar to those at the B N F M R A a2 except that they are fed with sea-water on a "oncethrough" basis. Water flow and air injection are controlled so that the jet impinges on the specimen (from a 2 m m nozzle positioned 2 m m from the surface) at a velocity of 4-6 m s -~ (15 ft s -x) with 3% air added in the form of large bubbles. Each rig accommodates twelve specimens which are in the form of half-section of a standard 1 in. o.d. condenser tube, 3 in. long. The normal period of test is 28 d; the specimens are rotated clockwise to adjacent specimen holders every working day to compensate for minor variations between jets. At the end of the test the specimens are washed, dried, weighed, and the dimensions of the pit determined. Type 316 WA and 316L WA specimens were prepared and tested so that the impingement region included the welded seams. After completing a standard test there was no sign of any damage due to impingement attack on any specimens. The only effect was a small rust-coloured area at the point of impingement; this was found to be fairly loose and only partially adherent and is presumed to have been deposited on the specimen surface from the sea-water which had picked up traces of Fe. There were no signs of crevice attack in the area of fixture to the specimen holder and the specimens showed no loss of weight. These observations suggested that the stainless steels offer very good resistance to inlet end impingement damage.
256
D.W.C.
BAKER,
W. E. HEATONand B. C. PATIENT
4~8 in
.
.
.
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-~
.
~ ]
l~6in,pitch(20
"
~-
SWG)
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WELDED:
T u b e P l a t e and T u b e s of S a m e S p e c i f i c a t i o n . 1 , 3 , 5 and 7 We[ded A n n e a l e d D r a w n and A n n e a l e d . 2, 4, 6 and 8 W'elded and A n n e a l e d , Z, 4, 5 and 7 W e l d e d on Both F a c e s .
(u)
EXPANDED:
A l l T u b e s F i t t e d by E x p a n s i o n . (a) 316 T u b e P l a t e Tubes 2, 3, 5 and 8 Type 316 Tubes 1,4, 6 and 7 Type 316L (b) 304 T u b e P l a t e Tubes Z, 3, 5 and 8 Tubes I, 4, 6 and 7
Fit]. 5.
T y p e 304 Type 304L
Tube/tube plate specimens for exposure in water boxes of operating condensers.
Evaluation of welded and expanded tube/tubeplate assemblies Tests in water boxes of operating condensers. In order to assess the behaviour of tube/tubeplate joints in stainless steel, experimental tubeplate assemblies were prepared and installed in the water boxes of condensers at the same station as used • for the substitute condenser tube trials. In the case of the austenitic steel welded tube/ tubeplate assemblies the plate and tubes were nominally of matching specification. Welding was carried out using a "Tubesealer" automatic argon arc welding equipment (British Oxygen Co. Ltd.) with H F arc initiation. Each plate contained eight tubes, four in the welded and annealed condition and four in the welded, annealed, drawn and annealed condition. Two tubes of each category (i.e. four in all) were welded on one face of the tubeplate only while the remaining tubes were cut to a length to match the thickness of the tubeplate and welded on both faces. The latter technique was designed to introduce residual longitudinal contraction stresses which might influence behaviour in service. The arrangement of the tubes in each assembly is shown in Fig. 5.
The evaluation of stainless steels
257
Similar austenitic steel test plates were made containing expanded tubes but in this case both "normal" and "low" C varieties of the steel tubes were fitted in either a "normal" or "low" C tubeplate because of the limited availability ot~ these plate materials. Test plates of each type, i.e. as listed below, were assembled in mild steel racks by means of insulated tie rods in the order 316L welded, 316 welded, 316/316L expanded, 304/304L welded and expanded, 304L welded, and 304 welded. The rack and exposed mild steel parts were painted with an anti-corrosion paint and then fitted by a suitable bracket to a convenient structural member in the water box of a condenser at each of the seven test sites. The appearance of a test plate before test and of an assembly after testing are shown in Fig. 6. After exposure the test plates were visually and destructively examined. The expanded tubes were sawn along their length and sprung out of their holes so that the interface could be examined for crevice type corrosion both at the front face of the plate where a "tight" joint should have been obtained and at the back where the expansion terminated within the thickness of the tubeplate forming a crevice. It is impracticable to record the detailed results of the examination of every feature of every weld in the present paper. However, the broad conclusions were that crevice corrosion occurred both under the tie-rod washers and at the interface between the expanded tubes and tubeplates in all materials at one or more of the test sites. P!tting corrosion of both the plates and the tubes also occurred in all the material categories. In some instances pitting tended to occur preferentially either in the heat affected zone of the weld or on the weld bead itself. Tests at Brighton Sea-water Corrosion Laboratory. In addition to the tests carried out in power station condensers other test plates were exposed to flowing sea-water in a facility designed for that purpose at Brighton Sea-water Corrosion Laboratory. 'The form of the test plates used is illustrated in Fig. 7. The tubes were fitted either by welding or by expanding, using a controlled torque tool. The amount of expansion was varied within the tubes of each plate to determine whether there was a level of expansion below which there was a danger of crevice corrosion at the interface. Apart from the expanded 304 and 304L tubes which were fitted in the same 304 plates, the tubes and plates were of the same nominal specification. Up to nine plates were connected in series using standard plastics flanges (to B.S. 10, 1962, Table " E " ) and 5-in. bore plastic piping (Durapipe). Neoprene rubber gaskets were fitted between the tubeplate surfaces and the flanges to make a watertight sea. "['he plates were spaced about 36 in. apart in order to minimize the disturbanee in flow. The circulating water was taken from the inlet culverts of Brighton " A " Power Station at a flow of 3500 gal/h (6.3 1 s -1) corresponding to a velocity of about 1.3 m s -1 (4 ft s -1) through the tubes. Again it is impracticable to present the detailed results but the following general observations may be made: (1) In all the tests the plates became heavily encrusted with marine growths and deposits, including barnacles and worms. Corrosion of the tubeplate surfaces occurred under some of the marine growths. (2) A ring of crevice corrosion occurred around the periphery of the exposed part of the plate where the gasket made contact. After testing the plates were scrubbed
258
D. W. C. BAKER,W. E. HEATONand B. C. PATIENT 1420 S.W.G.TUBES.
•
9"
•
L
TUBES:
1, 3, S a n d 7
Welded Annealed
TUBES:
2, 4 a n d 6
Welded and Annealed.
Drawn and Annealed•
For Welded Assemblies: TUBES:
I, a n d 2
TUBES:
3, 4, 6, a n d 7 W e l d e d
Welded
on o n e face. o n both faces.
I~G. 7. Form of test plate used for evaluation of tube/tubeplate joints at Brighton Sea-water Corrosion Laboratory. down and examined visually with the aid of a low power microscope. Sections were prepared from selected areas for metallographic examination. All the materials suffered pitting corrosion to a greater or lesser extent. In general, the expanded joints showed crevice corrosion of the interfaces. There was no evidence that the amount of expansion, which varied from 3 to 24% wall thinning, had affected the extent of crevice corrosion.
Exposure of specimens under deposit and crevice corrosion conditions In the initial series of tests, samples of each tubing category were mounted in an epoxy resin base together with a brass supporting rod as shown in Fig. 8. The mount was formed by casting the resin around the base of the tubes located vertically in a suitable circular mould. The specimens in each assembly comprised lengths of 20 SWG tubing in the following material categories:
The evaluation of stainless steels 304 WADA 304L WADA
316 WADA 316L WADA
304 WA 304L WA
259 316 WA 316L WA
Rubber bands were placed around each tube about 1 in. from the top of the tube to form a crevice-like interface. In later repeat tests stainless steel bands made from tubes of matching composition were used instead of rubber. Assemblies of this type were immersed under relatively stagnant conditions such as reserve cooling water tanks or circulating water culverts at eight of the stations where condenser trials were carried out, i.e. at Northfleet, Blackwall Point, Tilbury, South Denes, Stella South, Stella North, Ferrybridge " B " and Fleetwood. These tests again demonstrated that all varieties of stainless steel tested were susceptible to crevice attack both at matching metal/metal junctions and at metal/rubber junctions. In many instances, pitting corrosion was observed. Damage was most severe in the test plates exposed at Tilbury, South Denes, and Fleetwood. In one test at Fleetwood Power Station, one 316 WADA tube had suffered gross general corrosion and "chemical polishing" of the bore where it had been filled with deposit. Laboratory stress-corrosion and weld decay tests Accelerated laboratory stress-corrosion and weld decay tests were carried out to determine whether there was any difference in behaviour between the various categories of steel and whether the tube/tubeplate welds were likely to be susceptible to failure by either of these mechanisms. The test specimens used for both types of test were similar and comprised: (a) Lengths of plain tubing as received and after annealing at 1000°/1050°C. (b) Single tube/tubeplate welded specimens. (c) Tube/tubeplate specimens welded on both faces of the plate to introduce additional residual stresses. The plain tube specimens were prepared by carefully sawing through the tubing while it was clamped firmly in a jig designed to prevent distortion during cutting. It was found, during preliminary commissioning tests of the apparatus, that many fine cracks were generated at the sawn ends of the tubing and they were subsequently dressed by filing and grinding (to grade 600 paper) to obviate this preferential attack. Stress-corrosion tests. The initial stress-corrosion tests were carried out in boiling 42%MgC12. lz The specimen was removed from the apparatus every 24 h, washed in water, dried and examined with the aid of a hand-lens. The time taken for cracks to appear was noted. The tests were terminated after a total of 96 h exposure and sections taken for metallographic examination. Subsequently some tests were carried out at room temp. on welded specimens in a solution of 3 %NaC1 saturated with H~S. 14 (a) Plain tube tests. Although the behaviour of the plain tube specimens in the tests carried out in boiling 42%MGC12 solution was variable, it was found that in the "as received" condition all four varieties (316, 316L, 304 and 304L) were susceptible to cracking. Except in the case of the 304L WA tubing, which appeared to be particularly susceptible, annealing eliminated cracking although pitting attack occurred in most instances. The latter may be attributable to the superficial oxidation which occurred during the prior heat treatment of the specimens and impaired the general corrosion resistance of the material.
260
D.W.C. BAKER,W. E. HEATONand B. C. PATIENT
There was no evidence that the presence of the welded seam in the WA or W A D A specimens was preferentially susceptible to cracking. The cracks which did occur tended to lie parallel to the axis of the tubing and in some instance they extended the whole length of the specimen, the tube springing open confirming the presence of residual stresses in the hoop direction in the as-received tubing.
(b) Tube/tubeplate specimen tests. Tests were carried out in boiling MgC12 solution on the tube/tubeplate specimens in the same manner as those on the plain tube specimens. Again the response of the specimens differed in replicate tests in many instances. However, it was clear that all the material categories were susceptible. The incidence of cracking of the tubeplate tended to be greater in the immediate vicinity of the weld bead. Cracking of the tubes was also concentrated in many instances in the heat affected zone about 1-2 mm from the edge of the weld bead in the area where a circumferential band in the bore of the tube was discoloured by oxidation during welding. In a number of instances the cracking of this zone was such that the tube fractured and broke away during preparation of sections. There was no evidence that the specimens welded on both faces behaved differently from those welded on only one face. Metallographic examination of sections of the tube/tubeplate specimens showed that the cracking was transgranular in all cases, and did not appear to deviate on passing from one structural zone to another, e.g. from the tubeplate into the weld metal. In addition to the tests carried out in boiling MgCI 2 solution tests were also made in a 3%NaCI solution saturated with H2S at room temp. This solution was used by Phelps and Mears 14 in the course of a study of the relationship between composition and structure of stainless steels and their resistance to stress--corrosion. It was chosen since it approximated to that which might occur in practice at a polluted estuarine site and it was reported to have caused cracking of Type 304 steel in the cold-worked condition at room temp. Tests were carried out on 316 WA, 316 WADA, 316L WA, 316L WADA, 304 WA, 304 WADA, 304L WA, and 304L WADA, tube/tubeplate specimens welded on one and both faces respectively. The solution was used at room temp. and the concentration of sulphide was maintained by bubbling H2S gas through the solution for two 10-min periods each day. The tests were continued for 96 h and the specimen examined microscopically before and after sectioning. The only specimens which showed signs of significant deterioration were both types of welded 316 specimens with W A D A tubes. In these specimens cracks were observed both in the tubeplate and in the weld metal. The cracking was shallow and metallographic examination suggested that it was initiated at the surface of the tubeplate which contained evidence of cold work as a result of machining. Welddecay tests. The weld decay tests were carried out in a boiling H2SOJCuSO4.14 The specimens were cut from the tubes in the same manner as the stress-corrosion specimens and weighed. The apparatus was as used for the stress-corrosion tests, the flask containing 600 ml of the test solution. The solutions were brought to the boil and allowed to reflux gently for 72 h after which the specimens were removed, washed, dried and reweighed.
The evaluation of stainless steels
261
At least two tests were carried out on each type of tubing. In addition to weight-loss measurements the specimen were examined microscopically and mechanically tested by flattening and expansion tests. In the flattening tests the tube was flattened by hammer blows until the minimum dimension was reduced to one-half the original dia. In the expansion test a taper drift wiih an included angle of 45°was driven into the end of the tube until the dia. at the mouth was greater than the original dia. by at least 50°;. All of the samples withstood these tests without cracking. Some fine pitting occurred in all the specimens but it tended to be more marked in the 316 and 316L tubing than the 304 and 304L and greater in the welded and annealed tubing than in the welded, annealed, drawn and annealed tubing. Tests were also carried out on welded tube/tubeplate specimens both on those welded on one face only and those welded on both faces. No significant weight-loss measurements could be made because of disproportionate mass of the tubeplate compared with that of the tubing. None of the samples showed evidence of deterioration.
DISCUSSION The complete results of the field trials are not yet available but it is clear that all the materials (i.e. 316L, 316, 304L and 304) even in the preferred condition, i.e. welded, annealed, drawn and annealed, are susceptible to corrosion. This has ranged from superficial pitting exhibited by the tubes exposed, for example, at Bl'ackwall Point, to severe perforation and pitting suffered by the tubes at the estuarine sites at Tilbury, Northfleet, South Denes, Fleetwood and Uskmouth. It is also evident from the tests at the Brighton Sea-water Corrosion Laboratory that these materials are not suitable for service in sea-water cooled condensers. The Mo bearing steels (316 and 316L) have in general behaved better than the straight Cr-Ni steels but the difference, if any, has been one of degree rather than kind. One curious feature of the Tilbury tube trials was that in both tests all four different categories of tubing failed simultaneously after the same period of exposure. The fact that the tubing survived longer at Fleetwood and the Stella stations than at Tilbury, although the CI concentrations in the cooling waters are similar, may indicate that the levels of pollution and residual 02 are important in maintaining the integrity of the surface film on austenitic steels. At Northfleet and Brighton, tubes in the welded and annealed condition were also tested. In general, there is no evidence that the performance of the weld was markedly different from that of the parent material in the sense that failure occurred both at the welded seam and elsewhere. However, there were some pits that were located so precisely on the weld seam that it seems likely they were initiated by some local feature. The stainless steel tubes in the site trials, mounted in conventional condensers, would be expected to be slightly cathodic with respect to both the brass tubeplate and cast-iron water box. There was no evidence that this electrochemical effect had had any influence on the behaviour of the tubes or the tubeplates in the immediate vicinity of the stainless steel sample tubes.
262
D.W.C. BAKER,W. E. HEATONand B. C. PATIENT
An aspect of the tests carried out at Brighton Sea-water Corrosion Laboratory was the variability of behaviour of the materials. Exhaustive attempts were made to determine the reasons for the variable behaviour at Brighton by detailed chemical analyses, metallographic examination and hardness testing of the different materials. No correlation between the incidence of pitting corrosion and any structural or compositional feature could be found. The tube rig tests at Brighton, the tests of welded and expanded tubeplate assemblies and the test on tubes under relatively stagnant water conditions have all shown that the austenitic stainless steels are very sensitive to crevice corrosion both at interfaces between metal parts of matching composition and at interfaces between the steels and non-metallic materials, e.g. rubber and marine organisms. It was not possible to define any geometrical limits which offered immunity. The catastrophic damage suffered underneath neoprene gaskets in the tests in sea-water certainly show that once this mode of attack has started the clearance does not appear to be critical and can certainly be greater than 1 mm. At the other end of the scale the negligible clearance offered by a mechanically expanded joint did not prevent attack although it was less marked than in sliding fits. The form of pitting observed in the Brighton rigs (which carried no heat load) was so similar to the pitting observed from tubes in operational condensers that thermal effects were not considered to be playing any part in the corrosion processes. The jet impingement tests in flowing sea-water showed no susceptibility either of the 316 or 316L tube materials or welded seams to attack by impingement and this is supported by the absence of any impingement type damage in the condenser trials. In view of the performance of the tubing itself, the behaviour of welded joints is no longer crucial but the evidence suggests that welds in these materials are only marginally inferior to the parent materials in their resistance to pitting attack. The investigation has again emphasized the difficulty in carrying out meaningful corrosion tests on materials such as stainless steels which depend for their survival on the integrity of passive surface film. Many of the specimens examined in the present work have appeared, over the vast bulk of their surface, to be in as good condition as when they entered service but here and there serious pits and perforations have developed. Sometimes there has been some obvious correlation of the pit and a local feature, for example, an adherent marine organism, a discontinuity in the weld or a manufacturing defect in the tube; but in the majority of instances there is no residual evidence that the site of the pit was in any way different from the remainder of the surface. Furthermore, similar features and flaws have, in the majority of cases, not been the source of pitting. The laboratory stress-corrosion and weld decay tests showed little difference between the various material categories studied but there was some indication that the heat-affected zones of welds would prove the most sensitive to cracking if a stresscorrosion situation did arise in practice. In fact, cracking was not observed in any of the operational or rig trials. The reason why the performance of the stainless steels in these trials is inferior to that reported from the U.S.A. is uncertain but it may reflect a fundamental difference in the water conditions. Cleanliness of condenser tubes has always been emphasized in American publications on this subject and there have been rumours
The evaluation of stainless steels
263
o f serious failures by pitting a t t a c k in condensers which have been left filled with stagnant water. However, at Tilbury a test was carried out in one condenser o p e r a t i n g with T a p r o g g e o n - l o a d cleaning a n d the tubes p e r f o r m e d no better t h a n those in a neighbouring unit o p e r a t i n g without such facilities. There is no evidence t h a t these tubes were allowed to stand in stagnant water for any significant lengtla o f time a n d yet they failed in a few months. CONCLUSIONS (1) The austenitic stainless steel tubes p r o d u c e d by a T I G welding process in A1SI Types 316, 316L, 304 and 304L steels are unsuitable for c o n d e n s e r tube service in sea-water a n d polluted estuarine environments. (2) The austenitic steels are particularly susceptible to crevice c o r r o s i o n at interfaces such as r u b b e r - m e t a l joints and t u b e / t u b e p l a t e expansions in the cooling water environments studied. (3) A p a r t from crevice corrosion, the chief m o d e o f failure is by pitting c o r r o s i o n . The M o bearing steels (316 and 316L) are distinctly better than the straight C r - N i steels (304 a n d 304L) in this respect but are not immune. (4) The p e r f o r m a n c e o f weld in the austenitic stainless steels was n o t significantly different from parent tube or tubeplate material although there were situations where corrosion has initiated at a local feature o f a weld. (5) The Type 316 a n d 316L austenitic steels are resistant to i m p i n g e m e n t a t t a c k under the condition o f the jet i m p i n g e m e n t test using flowing sea-water and are superior to conventional non-ferrous condenser tube alloys in this respect. Acknowledgements--The authors would like to express their thanks to the many people who assisted
in the investigation: to the Station Superintendents at Northfleet, Tilbury, Blackwall Point, South Denes, Flcetwood, Ferrybridge, Stella South, Stella North and Uskmouth Power Stations and their respective staffs, particularly the Station Chemists; to their colleages in North Western, North Eastern and South Western Regions' Scientific Service Departments for assistance with the preparation of test-pieces and their exposure at the test sites. They would also like to acknowledge the contribution of the late Mr. W. Matthewman to the work at the Brighton Sea-water Corrosion Laboratory and to their colleagues at Cockfosters, particularly Mr. M. F. Jones and Mr. P. Mukerjee, who assisted with the examination of the test specimens. The paper is published by permission of Mr. R. A. Peddie, Director General, South Eastern Region, Central Electricity Generating Board. REFERENCES 1. H. J. VYHMALEK,27th Meeting of the American Power Conference, Chicago, Illinois (1965). 2. J. H. HARLOW,ASME-IEE Power Conference, San Francisco (1961). 3. J. R. MAURER, Basic Considerations in the Use of Stainless Steels in Modern Condenser Design. The International Nickel Power Conference, Estes Park, Colorado (August 1961). 4. N. A. LONG,Recent Operating Experiences with Stainless Steel Condenser Tubes. American Power Conference, Chicago (1966). 5. F. L. LAQOEand M. A. CORDOVl,Experiences with Stainless SteeI Surface Condemers in the U.S.A. Paper No. 12, International Nickel Power Conference, Lausanne (1967). 6. V. N. GULYAEVand M. I. LUZHNOV, 'The Choice of Materials for Condenser Tubes', Thermal Eng. 11 (3), 85 (1964). 7. R. GASPARINIand P. STURLA,Experience and Selection of ColMenser Materials for Power Plants. Paper No. 10, INCO Power Conference, Lausanne (1967). 8. British Standard No. 378. 9. G. SACHSand VAN HORN,Practical Metallurgy, p. 181. ASM, Cleveland, Ohio (1951). 10. Anon, Engineering, June 27, 1958, British Patent 700, 833.
264
D . W . C . BAKER,W. E. HEATON and B. C. PATIENT
W. MATTHEWMANand W. E. HEATON,Corros. Sci. 8, 453 (1968). R.. MAY and R.. W. DE VERE STACPOOLE,J. Inst. Metals 77, 351 (1958). M. A. SCHEIL et al., Weldingd. 22, 493/5 (1953). E. H. PHELPS and R. B. MEARS, The Effect o f Composition and Structure o f Stainless Steels upon Resistance to Stress Corrosion CracKing, I st International Congress on Metallic Corrosion, p. 319. Butterworths, London (1962). 15. J. H. G. MONEYPENNY, Stainless Iron andSteel, Vol. 1 (3rd Ed.), p. 99. Chapman & Hall, London (1951). 11. 12. 13. 14.