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Stress corrosion crack propagation in α-1 brass in ammoniacal solutions
Corrosion Science, 1974, e e l . 14, pp. 619 to 630. Pergamon Press. Printed in Great Britain
STRESS CORROSION CRACK P R O P A G A T I O N IN a-1 BRASS IN A M M O N I A C A L SOLUTIONS* G. M. SPARKESand J. C. SCULLY Department of Metallurgy, Houldsworth School of Applied Science, University of Leeds, Leeds LS2 9JT, England Abstract--Average stress corrosion crack velocities have been measured on 70Cu-30Zn brass as a function of pH. A maximum was found at pH 7.8 and increasing values at pH > 10. These are considered to correspond to borderline conditions for the formation of both soluble and insoluble species. Fractographic examination showed that intergranular cracking predominated both in cold worked bar and in annealed sheet at all values of pH up to 13.5 where transgranular cracking predominated. It is concluded that one mechanism of cracking is operative based upon localized dissolution associated with localizedenrichment of zincand that intergranular to transgranular transitions may result from high corrosion rate conditions. The tarnish film is considered to be of importance only in providing some protection in the neutral pH range. INTRODUCTION Tim FAmUREof a-brass by stress corrosion cracking in ammoniacal environments has been widely investigated over the last 30 years. The phenomenon has been th~ subject of numerous reviews. 1-8 Cracking is considered to occur in two different modes, intergranular and transgranular, depending mainly upon the pH of the solution, w6 but also upon the value of the potential and the degree of cold work.6 Such distinctions arose from the work of Mattsson 4 who distinguished two types of solution, tarnishing and non-tarnishing, which caused intergranular and transgranular cracking respectively. The tarnish film was considered to be Cu,O and this has been identified as an epitaxed layer. 7-g Mattsson employed potential/pH diagrams for pure copper and zinc in ammoniacal solutions in order to explain the possible equilibrium surface reactions that occur. The diagram for copper has since been modified by several authors. 7,1° That due to Hoar and Rothwell t° shows that two regions of pH exist where Cu20 formation can be expected, one between pH 3 and 7.3, the other at pH > 11. Their diagram was based upon 1 mol/l (NHa and N H +) and 0.05 mol/l of copper at 25°C as originally employed by Mattsson. Cracking in neutral solutions 4 and in strongly alkaline solutions (e.g. 15N NH3) a is intergranular and occurs only after the formation of a visible blackish tarnish film. Under open circuit conditions the presence of dissolved copper is necessary since it promotes tarnishing but under conditions of anodic polarization in ammoniacal solutions tarnishing occurs in solutions that contain no copper initially. B Considerable emphasis has been placed upon the role of the tarnish film in the cracking process. Its formation and fracture in alternating steps has been put forward *,8 as the main factor in the propagation stage and in determining the intergranular mode of cracking. In neutral tarnishing solutions the fracture surface is yellow, not black, n,~2 *Manuscript received 29 January 1974. 619
620
G. M. SPARKESand J. C. SCULLY
and although the invisible oxide has been identified u by electron diffraction as Cu20 the observation would suggest that at the time of formation of new crack surfaces there is little film present at the tip. Birley and Tromans ~2 have suggested that the formation of CuzO into the alloy surface could release pile-ups of dislocations which emerge and create fresh reactive metal. Hoar and Booker, 9 however, place the main emphasis upon dissolution. Microstrain of the ductile metal at grain boundaries breaks the brittle film and propagation occurs by a single stage yield-assisted dissolution model. Cracking occurs by the simultaneous anodic production of solid CuzO and soluble diamine (I) complex which is oxidized by the dissolved oxygen to the tetramine and which in turn is reduced, possibly on the oxide surface, providing a highly localized cathodic reactant. In support of the film rupture mechanism, claims have been made xa,x4 of fractographic evidence supporting intermittent propagation from visible striations and of a linear relationship between cracking rate and film growth rate. 15 In the work reported below measurements of crack velocity have been made as a function of pH and the fracture surfaces have been examined. It is concluded that the maximum crack propagation rate is associated with borderline conditions between film formation and dissolution and that the general association of a single separate form of fracture with either tarnishing or non-tarnishing conditions is not justified. A general single model is proposed. Cracking occurs by preferential dissolution of zinc rich regions and the tarnish film is of only secondary importance. EXPEKIMENTAL
METHOD
The experimental arrangement for measuring crack velocity was similar to that described previouslyla for work on a Cu-Be alloy. Single edge notched specimens 170 x 25 x 9.5 mm were loaded in a cantilever beam arrangement as shown in Fig. 1. The notch was 4.76 mm deep cut into the 25 nun depth dimension and it subtended an angle of 60°. Specimens were machined from cold-rolled plate of composition 69.9%Cu, 30.01%Zn, 0.022%Fe, 0.001%S by wt. and with a V H N of 184. The grain size was c a . 800 ~m in the rolling direction and c a . 150 ~m across the rolling direction. The solution was added to a small polyethylene beaker cut so that the specimen could be glued through it. The specimen sides were coated with a resistant lacquer so that only the notch surfaces were exposed to the solution. Specimens were cut from the plate with the longitudinal axis orthogonal to the rolling direction. Crack propagation occurred as a single unbranching crack propagating at right angles to the length and its progress was followed by a dial gauge or displacement transducer placed in position A or B in Fig. 1. The experimental stress intensity factor was calculated according to the formula of Brown. 14 No pre-crack was employed. Each test was conducted for up to 24 h. When cracking occurred (as it did for all solutions of pH > 1) it was initiated in under 60 rain from start of the test. This was determined from movements of the dial guage and from examination of specimen notch roots in preliminary experiments. The time of propagation was measured from when the dial gauge pointer started to move. At pH > 6 specimens were loaded initially to a Kvalue of 35MNm -312. Specimens broke when the crack length was c a . 6 mm corresponding to a Klx value of 67MNm -2/3. At pH < 6 because the crack velocity was low specimens were loaded initially to a
Stress corrosion crack propagation in a-brass in ammoniacal solutions
621
Transducer Specimen
vironmen?
Load \
I
I
I FIG. 1. Diagram of the stress corrosion testing rig. The displacement transducer or the dial gauge was placed either in position A or in position B. value of 48.4MNm -3/2. If failure had not occurred within 24 h the test was stopped by rapidly increasing the load on the specimen until overload failure occurred. The average velocity was then calculated by dividing the crack length determined fractographically by the measured time of crack propagation. Some experiments were done on annealed sheet, 0.5 m m thick, of the same composition and with a grain size of c a . 35 ~tm. The solutions used were (i) 0.88 mol/1 of ammonia with 0.05 mol/1 of copper with a pH range of 0.07-12, and (ii) 7.8 mol/l of ammonia with 0.05 mol/l of copper with a p H range of 10-13.5. The copper was added as CuSO4 and the p H was adjusted either by adding ammonia partly in the form of (NH4)2SO4 or by adding concentrated HzSO4 or N a O H . All the solutions were made up immediately before a test and any ageing occurred only during the test. During each test the pH of the solution changed slightly. In the solution containing 0.88 mol/1 of ammonia an increase in p H was measured in solutions o f p H < 7.5 and > 10. For values o f p H intermediate to those values a slight decrease was observed. In the more concentrated solutions an increase in p H was always observed. The increase in pH arose possibly from dissolution of copper: Cu + nNHa + ½02 + H20 = Cu(NH3)~ + + 2OH-. The decrease in p H arose possibly from the predominance of reactions similar to: Cu + 2NH~ - - e = Cu(NHa)~- + 2H +. Fracture surfaces were examined in the scanning electron microscope with no treatment after removal from the environment other than washing and drying. Surfaces for examination were obtained by cutting thin slices 3-5 m m thick off the fracture end of broken specimens.
G. M. SPARKESand J. C. SCULLY
622
EXPERIMENTAL RESULTS The results obtained are given under five separate headings: crack velocity measurements, fractographic observations, weight loss determination, tarnished film examination and potential measurements. Crack velocity measurements
The variation of average velocity with pH is shown in Fig. 2 in which each point shows the variation in pH during the experiment. For specimens where the crack velocity was > 76 nm/sec failure occurred in under 24 h. Cracking was observed over the entire p H range 1.06-13.5. No cracking was observed in solutions of p H 0.07 in tests lasting for 16 days. A record of the transducer movement on a specimen tested in a solution of p H 7.9 showed irregular crack propagation which from a compliance curve corresponded to rapid movement forward of 15-35 ~m approximately every 100 sec with this interval decreasing near to the point of overload fracture. This forward movement was much smaller than the grain size in that direction of ca. 800 vtm. The most notable features of Fig. 2 are the maximum in the neutral range of p H and the increasing rate towards the alkaline end of the diagram. At the alkaline end precipitation occurred in the solution with the lower ammonia content. This may have been a basic cupric sulphate but its identity was not determined. It is likely that the removal of copper from the solution would lower the velocity. Consequently the occurrence of an apparent maximum is indicative probably of the increasing amount of copper removed. In the stronger solution precipitation of a copper compound is prevented by the greater capacity of the solution to form complexes. The results in the neutral range are in general agreement with those of other workers who have measured minimum times to failure. Mattsson 4 showed that 63Cu-37Zn brass in a solution containing 1 mol/1 ammonia and 0.05 mol/l of copper 240 220 200 80 60 40
I00 Precipitation I I t-'- ---'-I
8O 6O 40 2O
tion I
2
5
4
.5
6
7
9
I0
II
17'
I
I
15
14
pH
FIG. 2. The effect of pH on the average stress corrosion crack velocity of brass specimens tested in two ammoniacal solutions. ( o ) 0.88 mol[1 of ammonia + 0.05 mol/l of copper. ([]) 7-8 mol[l of ammonia + 0"05 mol/l of copper.
Stress corrosion crack propagation in or-brass in ammoniacalsolutions
623
and aged for 48 h, developed cracks in the shortest time at pH values 7.2-7.3. Lahiri and Banerjee is found a minimum time to fracture at pH 6.6 in the same solution, and at pH of 6.7 and 6.8 with a copper concentration of 0.22 and 0.008 tool/1 of copper respectively. Johnson and Leja7 with an unaged solution containing 1.5 mol/l of ammonia and 0.04 mol/l of copper reported minimum time to failure of 69.5Cu30.5Zn brass at pH 6.5, although 7-7.1 has been reported 11 for the same solution by Tromans et al. The maximum in Fig. 2 at pH 7.8 is somewhat higher than those before in time to failure tests. The difference may arise from the differences in alloy and solution ageing treatments of the various results that have been cited. The results in the alkaline range ofpH in the stronger solutions are also in general agreement with reported times to failure of other workers and of the potential/pH diagram of Hoar and Rothwell3° They showed another Cu~O/soluble species boundary at pH > 11 in accord with the occurrence of tarnishing in alkaline solutions. They also reported times to failure in Mattsson solutions of pH 11.3 similar to those occurring at pH 7.3 with longer times at intermediate values. The observed trend agrees also with the work of Pugh and Westwood~e who showed that additions of NaOH to solutions containing 15 mol/l of ammonia could lower the time to failure by a factor of 10. Fractographic observations The mode of stress corrosion cracking was found to depend on pH as reported by other workers~,5,7,a.1° but the transition from primarily intergranular to primarily transgranular cracking occurred only in solutions of pH 13-5. At other pH values the cracking was primarily intergranular of varying amounts. This was a surprising result since the transition from one dominant mode to the other is usually8 presented as occurring on either side of the neutral tarnishing zone of pH. The proportion of the two modes was estimated from the fractographs and is shown in Table 1. Unlike Cu-Be alloys18 there was no marked change in the proportion with increasing stress intensity. A typical area consisting of intergranular cracking is shown in Fig. 3. This was obtained at pH 8 but was identical to intergranular fractures obtained at any pH. This general type of appearance with grains elongated in the direction of crack propagation arose from the way the specimens had been cut from the cold-rolled plate. Many slip lines have cut the surface during or after the fracture. These striations were observed on many of the intergranular surfaces obtained in both tarnishing and nontarnishing solutions. They have been reported by Lees, ~7 Birley and Tromans, 1~and Takano 5 and are a general characteristic of intergranular failures of metals accompanied by significant amounts of plastic strain. Thus striations may occur in more than one direction, they change directions across grain boundaries as shown in Fig. 4 and they are not affected by the removal of any tarnish film by inhibited acid. It is likely that it is such striations that have been interpreted as evidence for the tarnish film rupture mechanism,x8 The type of transgranular cracking obtained was observed to depend on the pH of the solution. At pH 7.9, where the cracking was mainly intergranular, transgranular cracking appeared to have propagated rapidly. An example is shown in Fig. 5. In the stronger ammonia solution ofpH 13 which also caused rapid and mainly transgranular
624
G. M. SPARKES a n d J. C. SCULLY TABLE 1. VARIATION OF PERCENTAGE INTERGRANULAR CRAcKnqG WITH p H OF THE ENVIRONMI~NT
Original pH of environment
% Intergranular cracking (approx.)
1 "06
4.6 5.18 6.5 7.2 8.0
9.0 9.4 10.0 (7-8 g mol/l) 11.1 11.3 12.5 (7.8 g reel/l) 13.5 (7.8 g reel/l)
60 70 80 80 80 75 75 75 70 65 60 20
Results taken from brass specimens broken in tests reported in Fig. 2. cracking similar fractures were observed as shown in Fig. 6. In both examples no obvious plane of cracking was observed and the many parallel markings were evidence of considerable plastic strain occurring during and possibly after the fracture. In solutions in which the crack propagation rate was slow, e.g. at pH < 7 and pH > 8.5 and < 10 large steps were observed on the transgranular surfaces so that the whole part of such surfaces appeared more crystallographic than those already described. An example of transgranular cracking observed at pH 9 is shown in Fig. 7 and at pH 11 in Fig. 8. In acidic solutions a similar type of transgranular cracking was observed. It appeared that the type of transgranular fracture was dependent upon the crack velocity. Thus it can be speculated that when the velocity is high interrupting slip steps do not cause much deviation as the crack tip passes through the grains whereas when the velocity is low there are long pauses in which the crack is arrested by plastic deformation and translation on to a parallel plane. Such a motion is suggested in the film made of propagation in single crystals in solutions in the same range of pH. x° For the purposes of comparison and in order to ensure that the morphology observed was not due to some unknown artefact in the bar some U-bend specimens of the annealed sheet were broken in solutions of pH 7.5 and 13. The fractographs obtained were very similar to those obtained in the cold-worked bar. At the higher pH predominantly transgranular cracking was obtained as shown in Fig. 9 in which a small amount ofintergranular fracture is apparent. In general this represented approximately 20 per cent of the fracture in the sheet the same as indicated in Table 1 for the bar specimens. In the lower pH solution of pH 7.5 predominantly intergranular cracking was observed and an example is shown in Fig. 10 in which the fracture is relatively featureless as is usual in low strain fractures of annealed alloys.
Weight loss determination To supplement the work done on the variation of crack propagation rate with the
FIG. 3.
An example of intergranular fracture obtained in a solution of pH 8. ( × 1170.) FIG. 4. An example of intergranular fracture obtained in a solution of pH 8. Slip lines with a separation of ca. 1-2 I~m are visible in two adjacent grains. In one grain a second set of lines is apparent. ( × 2300.) FIG. 5. An example of transgranular fracture obtained in a solution of pH 7.9. ( × 1300.) FIG. 6. An example of transgranular fracture obtained in a solution of pH 13. ( × 1300.)
[Facing page
624l
FIG. 7. An example of transgranular fracture obtained in a solutjon of pH 9. The large steps on the surface give an impression of a more crystallographic fracture than those of Figs. 5 and 6. On the right hand side is an adjacent region of intergranular cracking. ( x 1200.) FIG. 8. An example of transgranular fracture obtained in a solution of pH 11. This is similar to Fig. 7 and was generally characteristic of the slower form of propagation. ( x 1300.) FIG. 9. An example of transgranular fracture obtained in a solution of pH 13 obtained in annealed sheet. A small amount of intergranular fracture is visible. ( x 680.) FIG. 10. An example of the relatively featureless intergranular cracking observed in a thin sheet specimen in a solution of pH 7"5. ( x 1500.)
Stress corrosion crack propagation in a-brass in ammoniacal solutions
625
pH a series of weight loss tests was done on sheet specimens of 2500 mm 2 area in solutions containing 0.88 mol/l of ammonia and 0.05 mol/l of copper. A maximum in the weight loss was observed over the pH range 9.5-11.5 as was found in a Cu-Be alloy tested under identical conditions, is The results are shown in Table 2. It is of interest to note that this pH range did not coincide with the maximum in the average crack velocity shown in Fig. 2.
Tarnish film examination The tarnish film formed in a range of solutions of different pH values was identified and the rate of tarnish film formation, defined as the time for the specimen to turn completely black, was determined as a function of the pH of the environment. To determine the rate of tarnish film formation half of a fractured specimen (approx. 22 x 9.5 x 85 mm) was pickled in nitric acid for 30 s, washed in distilled water, rinsed in acetone and then dried. The specimen was then suspended in a 1-1. beaker containing ca. 800 ml of the required solution. The specimen was inspected every l0 rain for the first hour and then every hour up to l0 h when the test was stopped. If the specimen was not completely black after this time the solution was considered to be "non-tarnishing". The results for the solution of lower ammonia concentration are shown in part of TABLE 2. LOSS OF
EFFECT OF pH ON THE WEIGHT ft.-BRASS ~PECIMENS. TESTED IN
0'88 mol/1 AMMONIAAND 0'05 g atom/l COPPER
pH
Weight loss (mg)
7.0 7.1 7.2 7,3 7,41 7.5 7.61 7.71 7.82 7.93 8.06 8.11 9.49 9.53 10.0 10.21 10.39 10.58 10.8 10.86 10.97 11.2 11.4
3"0 3'5 3.9 4"1 6'3 7.6 7.9 8"8 10.7 14'8 16'0 17.0 19.0 22. I 22.9 23"0 22"8 22.5 21.0 17"1 16"0 15.0 12'8
Area of specimens = 2500 mm2. Time of immersion 20 min.
626
G. M. SPARKESand J. C. SCULLY
Table 3. At pH values < 5 dezincification was observed which is in agreement with Mattsson 4 who reported the same observation in a slightly more concentrated solution at pH values < 6.3. The highest rate of tarnishing was observed at pH 7.2 and 7.3 (20 rain) and also at pH 12-0 (45 rain). The borderline conditions for tarnishing appeared to be c a . 7.9-8.1 and 10.7-10.8. These results appear to show that the pH values that give maxima in the tarnishing rates, 7.2 and 12, do not coincide with the fastest rates of crack propagation which occur at pH values of 7.9 and 10.8. While the result at the lower pH value is unambiguous, at the higher pH there was a difference in the experimental conditions that were employed since the tarnishing tests lasted only for 20 min while the velocity tests lasted for 24 h during which time precipitation occurred which could be expected to lower the dissolved copper content and thereby lower the tarnishing capacity of the solution. A few specimens were tested in the stronger solution of ammonia at pH values 10-13.5. The rate of tarnish film formation was found to increase with increasing pH. After 20 min only the solution with pH value of 13.5 had caused complete tarnishing. After 5 h the solution of pH value 11 had caused tarnishing while over 10 h was required to achieve the same effect for pH 10 solutions. In this solution therefore the rate of tarnishing increased in the same sense as the rate of crack propagation. TABLE3.
RATE OF TARNISH FILM FORMATION A N D POTENTIAL MEASUREMENTS OF O.-BRASS SPECIMENS IN SOLUTIONS OF
0'88 tool/1 OF AMMO~A + 0"05tool/1 OF COPPER WITH pH 7-12"0 Original potential pH
Black
7.0 7.1 7'2 7"3 7.4 7'5 7'6 7.7 7.8 7"9 8.1 8.2 8'6 9.0 9.5 10"0 10"2 10.5 10.7 10"8 11"0 11"2 11"4 11"6 11'8 12.0
40 rain
30 rain 20 nun 20 mm 30 rain 40 min 40 mm 5O mill 50 nun 60 rain more than 10 h 10h 10 h 10h 10h 10h 10h 10 h 10h 9h 7h 5h 3h 2h 1½h 45 rain
(mV) E ~
Changein first hour
(mV)
224 215
+ 112 + 108
190
+ 40
162 150 118
+ 22 + 17
100
0
+
4
-20 -- 104
--
20 0
--
110
+
2
---
121 119
+ +
3 16
-- 113 -- 110 -- 107
+ + +
17 17 23
93
+ 60
Stress corrosion crack propagation in u-brass in ammoniacal solutions
627
Tarnish f i l m examination
The tarnish film was identified by X-ray diffraction of a powder sample. Specimens were pickled in HNOa and then immersed in the tarnishing solution for 24 h. After this time the tarnish layer was scraped off. The solutions employed and the results obtained are shown in Table 4. In solutions 2-5 all the patterns revealed Cu20 while in solution 1 ZnO and ZnSO4.3Zn(OH)2 were also detected. Cu20 has been observed and identified by many workers but not the other compounds. For example, Cu20 has been identified on brass exposed to Mattsson solutions at pH 2 and 10, 2o and at 3.8, 5.6 and 7.2. 9 Potential measurements
To determine the effect of the tarnish film on the rest potential the potential of specimens was measured as a function of time. The results are shown in Table 3 in which the comparison can be made with the tarnishing rates already described. In the neutral range of pH a potential rise occurred in tarnishing solutions which can be interpreted as indicating the relative protective nature of the film, whereas in nontarnishing solutions no appreciable rise in potential occurred. These results are in agreement with those of Hoar et al. 9,1° but somewhat more extensive. The rate of tarnish film formation appeared to be related to the rate of potential rise, both being at their highest at pH 7.2 and 7.3. In the alkaline range of pH the situation was different. At pH 12.5 the potential did not move from the original value of -- 234 mV (v. S.C.E.) even though the specimen was tarnishing slowly. This observatio~a is in agreement with the work of Green et al. 2~ with 15N ammonia which causes rapid tarnishing. They observed that the applied current required to sustain a given potential during film growth was constant and concluded that the film forming in these solutions is porous and non-protective. DISCUSSION
The highest rates of propagation in the less concentrated solution were observed at pH values near to the borderline conditions for the formation of both soluble species and insoluble tarnish film simultaneously. Although the structure of the film formed at neutral pH is not known, it grows parabolically as and it causes a rise in potential. It affords some degree of protection to the alloy. It appears therefore that a relatively simple mechanism is operative. Preferential anodic dissolution occurs at the crack tip and causes a maximum crack velocity when tarnishing occurs sufficiently rapidly to cover the sides of the crack and thus concentrate the dissolution at the crack tip. Any striations on the grain boundary surface arise from plastic deformation either during or immediately after the passage of the crack tip and would be rapidly tarnished. Such a cracking process would become slow when tarnishing occurred at a rate sufficient to cover the crack, or if dissolution was occurring on a larger area due to too slow a tarnishing rate so that the same total current resulted in a slower rate of corrosion at the crack tip. It appears that a similar mechanism of preferential anodic dissolution occurred in the stronger ammoniacal solution over the pH range 10-13.5. In these solutions the rate of crack propagation increased with increasing tarnishing rate. Little work has been done on the morphology of films forming in either of the two pH ranges but the evidence of Table 3 suggests that the two tarnish films have different properties. In the
G. M. SPARKESand J. C. SCULLY
628 TABLE 4.
IDENTIFICATION OF TARNISH FILMS FORMED ON BRASS IN FIVE SOLUTIONS OVER 24 h
Initial pH (1) (2) (3) (4) (5)
0.88 mol/l of ammonia + 0.05 mol/l of copper 0.88 mol/l of ammonia + 0.05 mol/l of copper 0"88 mol/l of ammonia + 0"05 mol/l of copper 7"8 mol/l of ammonia + 0.05 mol/l of copper 7.8 mol/l of ammonia + 0.05 mol/l of copper d values
CusO
8.14 10"83 11'2 12.5 13.5
ZnSO,.3Zn(OH)24HzO
Final pH 8.17 11"06 11"22
12.6 13"7 ZnO
Solution (1) pH 8'14
9"96(100) 5"80 (20) 5.15 (30) 4.17 (30) 3.85 (10) 3.45 (15) 3.22 (40) 3"11 (30) 3.01 (30) 2.72 (40) 2.60 (45) 2.41(100) 2.11(100) 1.82 (20) 1.65 (10) 1.56 (20) 1.49 (30) 1"29 (30)
10"0 (100) 5"91 (11) 5.11 (9) 4.19 (40) 3.83 (9) 3.61 (3) 3.28 (11) 3"I1 (9) 3.02 (9) 2.75 (90) 2.65 (35) 2.46(100) 2.13 (37)
2.09 (10) 1.89 (3) 1.62 (40) 1-56 (13)
1.51 (27) 1"28 (17)
Sohaion (2) pH 10.83
2.90 (30) 2.43(100) 2.11 (80) 1.51 (70) 1.27 (40) 1.25 (10)
3.02 (9) 2.46(100) 2.13 (37) 1.51 (27) 1-28 (17) 1.23 (4)
Solution (3) pH 11"2
3"03 (20) 2"47(100) 2"13 (80) 1.50 (70) 1.29 (20)
3.02 (9) 2.46(100) 2'13 (37) 1.51 (27) 1.28 (17)
Solution (4) pH 12.5
2.99 (25) 2.44(100) 2.12 (60) 1.50 (50)
3.02 (9) 2.46(100) 2.13 (37) 1.51 (27)
Solution (5) pH 13"5
2"98 (10) 2.45(100) 2"11 (80) 1"49 (50) 1.27 (20)
2.60 (56) 2.47(100)
3.02 (9) 2.46(100) 2"13 (37) 1.51 (27) 1.28 (17)
Figures in brackets refer to relative intensities.
1.47 (35)
Stress corrosion crack propagation in a-brass in ammoniacal solutions
629
alkaline solution the film is non-protective. The growth rate is linear 21 and the preferential corrosion of grain boundaries to considerable depth in unstressed specimens 3 in such solutions can be taken as evidence for the porous and, general nonprotective characteristics of such tarnish films. Intergranular corrosion and cracking have been explained 3 as arising from the segregation of zinc to grain boundary regions. Transgranular cracking has been attributed 22 to the reactivity of dislocation pile-ups where some solute segregation may have occurred. The possible role of zinc is to dissolve preferentially giving rise to a high density of copper kink sites. 23 The path that the crack takes will be determined by these considerations and also by the properties of the film. Where there is either no tarnishing, very slow tarnishing or the formation of porous non-protective tarnish films, there will be two possible paths for the crack to follow with the preference being dependent upon the relative reactivity of the two zinc-rich regions. The intergranular path is pre-existing, evidence for which is shown by the grain boundary penetration of non-stressed specimens, s whereas the transgranular path is brought into existence as an effect of the applied stress. The intergranular one is likely to be preferred, particularly at low plastic strain where the density of dislocations will be low and where protective tarnish films form on grain surfaces, although as Table 1 indicates, such influences are not complete. Increasing amounts of plastic deformation will tend to break up grain boundary regions and might be expected to increase the amount of transgranular cracking but no such changes were observed in the cold-worked bar material nor in the annealed sheet. A transition to transgranular mode can be expected only when the reactivity of transgranular features is comparable to that of grain boundary material. Even when this "comparable reactivity" is established the grain boundaries will continue to remain highly reactive and any anodic current will be distributed between these and the transgranular features. From these rather simple considerations of a complicated process it could be speculated that a high corrosion rate may be important in determining the transition from intergranular to transgranular cracking in addition to the absence of a protective tarnish film. It is a characteristic of transgranular cracking that it is accompanied by evidence of noticeable increased rates of corrosion, e.g. wide cracks with many small branches. 9 Consistent with this is the observation that intergranular to transgranular transition is promoted by anodic polarization and reduced by cathodic polarization. 6 Transgranular cracking occurs at pH values of pH 13.5 for which the weight loss is higher a than for any of the solutions reported in Table 2. Evidence of this work is that intergranular cracking is the dominant form of fracture except at very high pH and that the tarnish film serves to suppress the occurrence of transgranular cracking when it affords some degree of protection to the surface on which it forms. In neutral solutions the tip of the crack surface is yellow and there is no need therefore to involve the fracture of a protective black film. In alkaline solutions the unprotective nature of the black film also makes such fracture unimportant and therefore unnecessary. Mattsson 4 reported a simple distinction between the fracture effects of tarnishing and non-tarnishing solutions. The former caused intergranular fracture while the latter caused transgranular fracture. Such ideas have been supported by other workers, s,5 It is not clear where the difference in observed effects can be explained.
630
G. M. SPARKESand J. C. SCULLY
The cold w o r k in the b a r c a n n o t be involved since similar fractures and results were observed on the experiments in neutral a n d highly alkaline solutions on the annealed sheet. T h e fractures observed on brass are d e p e n d e n t u p o n the degree o f cold work, 9 potential s a n d c o m p o s i t i o n , b o t h o f the brass a n d o f the solution. I n the m a t e r i a l examined the fracture was mainly i n t e r g r a n u l a r and the f o r m a t i o n o f a tarnish film was not associated with the exclusive occurrence o f i n t e r g r a n u l a r cracking since while a high p r o p o r t i o n o f such cracking occurred in neutral solutions, in the high alkaline tarnishing solutions cracking was mainly transgranular. CONCLUSION Since no exclusive form o f fracture has been observed for any value o f p H within the wide range that has been studied, it is suggested t h a t p e r h a p s only one m e c h a n i s m o f fracture is operative in which localized dissolution occurs. This is t h o u g h t to arise from local concentrations o f zinc a t o m s at grain b o u n d a r i e s or in dislocations a l t h o u g h evidence for this has n o t been provided. T h e specific cause o f the variations in the average velocity has n o t been elucidated a n d would a p p e a r to d e p e n d u p o n a closer study o f velocity as a function o f stress intensity factor, potential, p H a n d a m m o n i a a n d dissolved c o p p e r content. This w o r k is now in progress 2~ and will be r e p o r t e d later.
Acknowledgements--One of the authors (G.M.S.) is grateful to the Science Research Council for a studentship for the whole of the period in which this work was carried out.
REFERENCES 1. A. R. BAILEY,Met. Rev. 6, 101 (1961). 2. E. N. PUOH, Environment-Sensitive Mechanical Behavior (Eds. A. R.. C. WESTWOODand N. S.
STOLOn), p. 351. Gordon & Breach, New York (1966). 3. E. N. PtrGH, The Theory o f Stress Corrosion Cracking in Alloys (Ed. J. C. SCtrLLY),p. 418 .NATO, Brussels (1971). 4. E. MATTSSON, Electrochim. Acta. 3, 279 (1961). 5. M. TAKANO,Corros. Sci. 813, 11 (1971). 6. S. C. SIRCAR,U. K. CHATTERJEE,M. ZAMINand H. G. VIJAYENDRA,Corros. Sci. 12, 217 (1972) 7. H. E. JOHrqSONand J. LFaA, Corrosion 22, 178 (1966). 8. A. J. FORTYand P. HUMaLE,Phil. Mag. 8, 247 (1963). 9. T. P. HOARand C. J. L. BOOKER, Corros. Sci. 5, 821 (1965). 10. T. P. HOAR and G. P. ROTHWELL,Electrochim. Acta. 15, 1037 (1970). 11. D. TROM~a~S,N. A. DOWDS and J. LEJA, Fundamental Aspects o f Stress Corrosion Cracking (Eds. R. W. STAEHLE,A. J. FORTYand D. VAN ROOYEN), p. 154. N.A.C.E., Houston (1969). 12. S. S. BxPa.EYand D. TROMANS,Corrosion 27, 297 (1971). 13. G. M. SPARK~ and J. C. SCULLY, Corros. Sci. 11,641 (1971). 14. B. F. BROWN, Mat. Res. Stand. 6, 129 (1966). 15. A. K. L.~HRI and T. BANmOE~, Corros. Sci. 8, 895 (1968). 16. E. N. POOH and A. R. C. W~TWOOD,Phil. Mag. 13, 121 (1966). 17. D. J. LEES, Corros. Sci. 12, 811 (1972). 18. A. J. McEVILY and A. P. BOND,J. Electrochem. Soc. 112, 131 (1965). 19. C. EDEt.EANU,Physical Metallurgy o f Stress Corrosion Fracture (Ed. T. N. RI-IODIN),p. 97. Interscience, New York (1960). 20. M. T ~ N o and S. SmMOD~a~, Corros. Sci. 8, 55 (1968). 21. J. A. S. GREEN,H. D. MrNOELBm~Oand H. T. YOLKEN,J. electrochem. Soc. 117, 433 (1970). 22. D. THOMASand J. NUTTING, Corrosion 21, 143 (1965). 23. M. Kr.~a~ANIand J. C. SCuLLY(work in progress).
STRESS CORROSION CRACK P R O P A G A T I O N IN a-1 BRASS IN A M M O N I A C A L SOLUTIONS* G. M. SPARKESand J. C. SCULLY Department of Metallurgy, Houldsworth School of Applied Science, University of Leeds, Leeds LS2 9JT, England Abstract--Average stress corrosion crack velocities have been measured on 70Cu-30Zn brass as a function of pH. A maximum was found at pH 7.8 and increasing values at pH > 10. These are considered to correspond to borderline conditions for the formation of both soluble and insoluble species. Fractographic examination showed that intergranular cracking predominated both in cold worked bar and in annealed sheet at all values of pH up to 13.5 where transgranular cracking predominated. It is concluded that one mechanism of cracking is operative based upon localized dissolution associated with localizedenrichment of zincand that intergranular to transgranular transitions may result from high corrosion rate conditions. The tarnish film is considered to be of importance only in providing some protection in the neutral pH range. INTRODUCTION Tim FAmUREof a-brass by stress corrosion cracking in ammoniacal environments has been widely investigated over the last 30 years. The phenomenon has been th~ subject of numerous reviews. 1-8 Cracking is considered to occur in two different modes, intergranular and transgranular, depending mainly upon the pH of the solution, w6 but also upon the value of the potential and the degree of cold work.6 Such distinctions arose from the work of Mattsson 4 who distinguished two types of solution, tarnishing and non-tarnishing, which caused intergranular and transgranular cracking respectively. The tarnish film was considered to be Cu,O and this has been identified as an epitaxed layer. 7-g Mattsson employed potential/pH diagrams for pure copper and zinc in ammoniacal solutions in order to explain the possible equilibrium surface reactions that occur. The diagram for copper has since been modified by several authors. 7,1° That due to Hoar and Rothwell t° shows that two regions of pH exist where Cu20 formation can be expected, one between pH 3 and 7.3, the other at pH > 11. Their diagram was based upon 1 mol/l (NHa and N H +) and 0.05 mol/l of copper at 25°C as originally employed by Mattsson. Cracking in neutral solutions 4 and in strongly alkaline solutions (e.g. 15N NH3) a is intergranular and occurs only after the formation of a visible blackish tarnish film. Under open circuit conditions the presence of dissolved copper is necessary since it promotes tarnishing but under conditions of anodic polarization in ammoniacal solutions tarnishing occurs in solutions that contain no copper initially. B Considerable emphasis has been placed upon the role of the tarnish film in the cracking process. Its formation and fracture in alternating steps has been put forward *,8 as the main factor in the propagation stage and in determining the intergranular mode of cracking. In neutral tarnishing solutions the fracture surface is yellow, not black, n,~2 *Manuscript received 29 January 1974. 619
620
G. M. SPARKESand J. C. SCULLY
and although the invisible oxide has been identified u by electron diffraction as Cu20 the observation would suggest that at the time of formation of new crack surfaces there is little film present at the tip. Birley and Tromans ~2 have suggested that the formation of CuzO into the alloy surface could release pile-ups of dislocations which emerge and create fresh reactive metal. Hoar and Booker, 9 however, place the main emphasis upon dissolution. Microstrain of the ductile metal at grain boundaries breaks the brittle film and propagation occurs by a single stage yield-assisted dissolution model. Cracking occurs by the simultaneous anodic production of solid CuzO and soluble diamine (I) complex which is oxidized by the dissolved oxygen to the tetramine and which in turn is reduced, possibly on the oxide surface, providing a highly localized cathodic reactant. In support of the film rupture mechanism, claims have been made xa,x4 of fractographic evidence supporting intermittent propagation from visible striations and of a linear relationship between cracking rate and film growth rate. 15 In the work reported below measurements of crack velocity have been made as a function of pH and the fracture surfaces have been examined. It is concluded that the maximum crack propagation rate is associated with borderline conditions between film formation and dissolution and that the general association of a single separate form of fracture with either tarnishing or non-tarnishing conditions is not justified. A general single model is proposed. Cracking occurs by preferential dissolution of zinc rich regions and the tarnish film is of only secondary importance. EXPEKIMENTAL
METHOD
The experimental arrangement for measuring crack velocity was similar to that described previouslyla for work on a Cu-Be alloy. Single edge notched specimens 170 x 25 x 9.5 mm were loaded in a cantilever beam arrangement as shown in Fig. 1. The notch was 4.76 mm deep cut into the 25 nun depth dimension and it subtended an angle of 60°. Specimens were machined from cold-rolled plate of composition 69.9%Cu, 30.01%Zn, 0.022%Fe, 0.001%S by wt. and with a V H N of 184. The grain size was c a . 800 ~m in the rolling direction and c a . 150 ~m across the rolling direction. The solution was added to a small polyethylene beaker cut so that the specimen could be glued through it. The specimen sides were coated with a resistant lacquer so that only the notch surfaces were exposed to the solution. Specimens were cut from the plate with the longitudinal axis orthogonal to the rolling direction. Crack propagation occurred as a single unbranching crack propagating at right angles to the length and its progress was followed by a dial gauge or displacement transducer placed in position A or B in Fig. 1. The experimental stress intensity factor was calculated according to the formula of Brown. 14 No pre-crack was employed. Each test was conducted for up to 24 h. When cracking occurred (as it did for all solutions of pH > 1) it was initiated in under 60 rain from start of the test. This was determined from movements of the dial guage and from examination of specimen notch roots in preliminary experiments. The time of propagation was measured from when the dial gauge pointer started to move. At pH > 6 specimens were loaded initially to a Kvalue of 35MNm -312. Specimens broke when the crack length was c a . 6 mm corresponding to a Klx value of 67MNm -2/3. At pH < 6 because the crack velocity was low specimens were loaded initially to a
Stress corrosion crack propagation in a-brass in ammoniacal solutions
621
Transducer Specimen
vironmen?
Load \
I
I
I FIG. 1. Diagram of the stress corrosion testing rig. The displacement transducer or the dial gauge was placed either in position A or in position B. value of 48.4MNm -3/2. If failure had not occurred within 24 h the test was stopped by rapidly increasing the load on the specimen until overload failure occurred. The average velocity was then calculated by dividing the crack length determined fractographically by the measured time of crack propagation. Some experiments were done on annealed sheet, 0.5 m m thick, of the same composition and with a grain size of c a . 35 ~tm. The solutions used were (i) 0.88 mol/1 of ammonia with 0.05 mol/1 of copper with a pH range of 0.07-12, and (ii) 7.8 mol/l of ammonia with 0.05 mol/l of copper with a p H range of 10-13.5. The copper was added as CuSO4 and the p H was adjusted either by adding ammonia partly in the form of (NH4)2SO4 or by adding concentrated HzSO4 or N a O H . All the solutions were made up immediately before a test and any ageing occurred only during the test. During each test the pH of the solution changed slightly. In the solution containing 0.88 mol/1 of ammonia an increase in p H was measured in solutions o f p H < 7.5 and > 10. For values o f p H intermediate to those values a slight decrease was observed. In the more concentrated solutions an increase in p H was always observed. The increase in pH arose possibly from dissolution of copper: Cu + nNHa + ½02 + H20 = Cu(NH3)~ + + 2OH-. The decrease in p H arose possibly from the predominance of reactions similar to: Cu + 2NH~ - - e = Cu(NHa)~- + 2H +. Fracture surfaces were examined in the scanning electron microscope with no treatment after removal from the environment other than washing and drying. Surfaces for examination were obtained by cutting thin slices 3-5 m m thick off the fracture end of broken specimens.
G. M. SPARKESand J. C. SCULLY
622
EXPERIMENTAL RESULTS The results obtained are given under five separate headings: crack velocity measurements, fractographic observations, weight loss determination, tarnished film examination and potential measurements. Crack velocity measurements
The variation of average velocity with pH is shown in Fig. 2 in which each point shows the variation in pH during the experiment. For specimens where the crack velocity was > 76 nm/sec failure occurred in under 24 h. Cracking was observed over the entire p H range 1.06-13.5. No cracking was observed in solutions of p H 0.07 in tests lasting for 16 days. A record of the transducer movement on a specimen tested in a solution of p H 7.9 showed irregular crack propagation which from a compliance curve corresponded to rapid movement forward of 15-35 ~m approximately every 100 sec with this interval decreasing near to the point of overload fracture. This forward movement was much smaller than the grain size in that direction of ca. 800 vtm. The most notable features of Fig. 2 are the maximum in the neutral range of p H and the increasing rate towards the alkaline end of the diagram. At the alkaline end precipitation occurred in the solution with the lower ammonia content. This may have been a basic cupric sulphate but its identity was not determined. It is likely that the removal of copper from the solution would lower the velocity. Consequently the occurrence of an apparent maximum is indicative probably of the increasing amount of copper removed. In the stronger solution precipitation of a copper compound is prevented by the greater capacity of the solution to form complexes. The results in the neutral range are in general agreement with those of other workers who have measured minimum times to failure. Mattsson 4 showed that 63Cu-37Zn brass in a solution containing 1 mol/1 ammonia and 0.05 mol/l of copper 240 220 200 80 60 40
I00 Precipitation I I t-'- ---'-I
8O 6O 40 2O
tion I
2
5
4
.5
6
7
9
I0
II
17'
I
I
15
14
pH
FIG. 2. The effect of pH on the average stress corrosion crack velocity of brass specimens tested in two ammoniacal solutions. ( o ) 0.88 mol[1 of ammonia + 0.05 mol/l of copper. ([]) 7-8 mol[l of ammonia + 0"05 mol/l of copper.
Stress corrosion crack propagation in or-brass in ammoniacalsolutions
623
and aged for 48 h, developed cracks in the shortest time at pH values 7.2-7.3. Lahiri and Banerjee is found a minimum time to fracture at pH 6.6 in the same solution, and at pH of 6.7 and 6.8 with a copper concentration of 0.22 and 0.008 tool/1 of copper respectively. Johnson and Leja7 with an unaged solution containing 1.5 mol/l of ammonia and 0.04 mol/l of copper reported minimum time to failure of 69.5Cu30.5Zn brass at pH 6.5, although 7-7.1 has been reported 11 for the same solution by Tromans et al. The maximum in Fig. 2 at pH 7.8 is somewhat higher than those before in time to failure tests. The difference may arise from the differences in alloy and solution ageing treatments of the various results that have been cited. The results in the alkaline range ofpH in the stronger solutions are also in general agreement with reported times to failure of other workers and of the potential/pH diagram of Hoar and Rothwell3° They showed another Cu~O/soluble species boundary at pH > 11 in accord with the occurrence of tarnishing in alkaline solutions. They also reported times to failure in Mattsson solutions of pH 11.3 similar to those occurring at pH 7.3 with longer times at intermediate values. The observed trend agrees also with the work of Pugh and Westwood~e who showed that additions of NaOH to solutions containing 15 mol/l of ammonia could lower the time to failure by a factor of 10. Fractographic observations The mode of stress corrosion cracking was found to depend on pH as reported by other workers~,5,7,a.1° but the transition from primarily intergranular to primarily transgranular cracking occurred only in solutions of pH 13-5. At other pH values the cracking was primarily intergranular of varying amounts. This was a surprising result since the transition from one dominant mode to the other is usually8 presented as occurring on either side of the neutral tarnishing zone of pH. The proportion of the two modes was estimated from the fractographs and is shown in Table 1. Unlike Cu-Be alloys18 there was no marked change in the proportion with increasing stress intensity. A typical area consisting of intergranular cracking is shown in Fig. 3. This was obtained at pH 8 but was identical to intergranular fractures obtained at any pH. This general type of appearance with grains elongated in the direction of crack propagation arose from the way the specimens had been cut from the cold-rolled plate. Many slip lines have cut the surface during or after the fracture. These striations were observed on many of the intergranular surfaces obtained in both tarnishing and nontarnishing solutions. They have been reported by Lees, ~7 Birley and Tromans, 1~and Takano 5 and are a general characteristic of intergranular failures of metals accompanied by significant amounts of plastic strain. Thus striations may occur in more than one direction, they change directions across grain boundaries as shown in Fig. 4 and they are not affected by the removal of any tarnish film by inhibited acid. It is likely that it is such striations that have been interpreted as evidence for the tarnish film rupture mechanism,x8 The type of transgranular cracking obtained was observed to depend on the pH of the solution. At pH 7.9, where the cracking was mainly intergranular, transgranular cracking appeared to have propagated rapidly. An example is shown in Fig. 5. In the stronger ammonia solution ofpH 13 which also caused rapid and mainly transgranular
624
G. M. SPARKES a n d J. C. SCULLY TABLE 1. VARIATION OF PERCENTAGE INTERGRANULAR CRAcKnqG WITH p H OF THE ENVIRONMI~NT
Original pH of environment
% Intergranular cracking (approx.)
1 "06
4.6 5.18 6.5 7.2 8.0
9.0 9.4 10.0 (7-8 g mol/l) 11.1 11.3 12.5 (7.8 g reel/l) 13.5 (7.8 g reel/l)
60 70 80 80 80 75 75 75 70 65 60 20
Results taken from brass specimens broken in tests reported in Fig. 2. cracking similar fractures were observed as shown in Fig. 6. In both examples no obvious plane of cracking was observed and the many parallel markings were evidence of considerable plastic strain occurring during and possibly after the fracture. In solutions in which the crack propagation rate was slow, e.g. at pH < 7 and pH > 8.5 and < 10 large steps were observed on the transgranular surfaces so that the whole part of such surfaces appeared more crystallographic than those already described. An example of transgranular cracking observed at pH 9 is shown in Fig. 7 and at pH 11 in Fig. 8. In acidic solutions a similar type of transgranular cracking was observed. It appeared that the type of transgranular fracture was dependent upon the crack velocity. Thus it can be speculated that when the velocity is high interrupting slip steps do not cause much deviation as the crack tip passes through the grains whereas when the velocity is low there are long pauses in which the crack is arrested by plastic deformation and translation on to a parallel plane. Such a motion is suggested in the film made of propagation in single crystals in solutions in the same range of pH. x° For the purposes of comparison and in order to ensure that the morphology observed was not due to some unknown artefact in the bar some U-bend specimens of the annealed sheet were broken in solutions of pH 7.5 and 13. The fractographs obtained were very similar to those obtained in the cold-worked bar. At the higher pH predominantly transgranular cracking was obtained as shown in Fig. 9 in which a small amount ofintergranular fracture is apparent. In general this represented approximately 20 per cent of the fracture in the sheet the same as indicated in Table 1 for the bar specimens. In the lower pH solution of pH 7.5 predominantly intergranular cracking was observed and an example is shown in Fig. 10 in which the fracture is relatively featureless as is usual in low strain fractures of annealed alloys.
Weight loss determination To supplement the work done on the variation of crack propagation rate with the
FIG. 3.
An example of intergranular fracture obtained in a solution of pH 8. ( × 1170.) FIG. 4. An example of intergranular fracture obtained in a solution of pH 8. Slip lines with a separation of ca. 1-2 I~m are visible in two adjacent grains. In one grain a second set of lines is apparent. ( × 2300.) FIG. 5. An example of transgranular fracture obtained in a solution of pH 7.9. ( × 1300.) FIG. 6. An example of transgranular fracture obtained in a solution of pH 13. ( × 1300.)
[Facing page
624l
FIG. 7. An example of transgranular fracture obtained in a solutjon of pH 9. The large steps on the surface give an impression of a more crystallographic fracture than those of Figs. 5 and 6. On the right hand side is an adjacent region of intergranular cracking. ( x 1200.) FIG. 8. An example of transgranular fracture obtained in a solution of pH 11. This is similar to Fig. 7 and was generally characteristic of the slower form of propagation. ( x 1300.) FIG. 9. An example of transgranular fracture obtained in a solution of pH 13 obtained in annealed sheet. A small amount of intergranular fracture is visible. ( x 680.) FIG. 10. An example of the relatively featureless intergranular cracking observed in a thin sheet specimen in a solution of pH 7"5. ( x 1500.)
Stress corrosion crack propagation in a-brass in ammoniacal solutions
625
pH a series of weight loss tests was done on sheet specimens of 2500 mm 2 area in solutions containing 0.88 mol/l of ammonia and 0.05 mol/l of copper. A maximum in the weight loss was observed over the pH range 9.5-11.5 as was found in a Cu-Be alloy tested under identical conditions, is The results are shown in Table 2. It is of interest to note that this pH range did not coincide with the maximum in the average crack velocity shown in Fig. 2.
Tarnish film examination The tarnish film formed in a range of solutions of different pH values was identified and the rate of tarnish film formation, defined as the time for the specimen to turn completely black, was determined as a function of the pH of the environment. To determine the rate of tarnish film formation half of a fractured specimen (approx. 22 x 9.5 x 85 mm) was pickled in nitric acid for 30 s, washed in distilled water, rinsed in acetone and then dried. The specimen was then suspended in a 1-1. beaker containing ca. 800 ml of the required solution. The specimen was inspected every l0 rain for the first hour and then every hour up to l0 h when the test was stopped. If the specimen was not completely black after this time the solution was considered to be "non-tarnishing". The results for the solution of lower ammonia concentration are shown in part of TABLE 2. LOSS OF
EFFECT OF pH ON THE WEIGHT ft.-BRASS ~PECIMENS. TESTED IN
0'88 mol/1 AMMONIAAND 0'05 g atom/l COPPER
pH
Weight loss (mg)
7.0 7.1 7.2 7,3 7,41 7.5 7.61 7.71 7.82 7.93 8.06 8.11 9.49 9.53 10.0 10.21 10.39 10.58 10.8 10.86 10.97 11.2 11.4
3"0 3'5 3.9 4"1 6'3 7.6 7.9 8"8 10.7 14'8 16'0 17.0 19.0 22. I 22.9 23"0 22"8 22.5 21.0 17"1 16"0 15.0 12'8
Area of specimens = 2500 mm2. Time of immersion 20 min.
626
G. M. SPARKESand J. C. SCULLY
Table 3. At pH values < 5 dezincification was observed which is in agreement with Mattsson 4 who reported the same observation in a slightly more concentrated solution at pH values < 6.3. The highest rate of tarnishing was observed at pH 7.2 and 7.3 (20 rain) and also at pH 12-0 (45 rain). The borderline conditions for tarnishing appeared to be c a . 7.9-8.1 and 10.7-10.8. These results appear to show that the pH values that give maxima in the tarnishing rates, 7.2 and 12, do not coincide with the fastest rates of crack propagation which occur at pH values of 7.9 and 10.8. While the result at the lower pH value is unambiguous, at the higher pH there was a difference in the experimental conditions that were employed since the tarnishing tests lasted only for 20 min while the velocity tests lasted for 24 h during which time precipitation occurred which could be expected to lower the dissolved copper content and thereby lower the tarnishing capacity of the solution. A few specimens were tested in the stronger solution of ammonia at pH values 10-13.5. The rate of tarnish film formation was found to increase with increasing pH. After 20 min only the solution with pH value of 13.5 had caused complete tarnishing. After 5 h the solution of pH value 11 had caused tarnishing while over 10 h was required to achieve the same effect for pH 10 solutions. In this solution therefore the rate of tarnishing increased in the same sense as the rate of crack propagation. TABLE3.
RATE OF TARNISH FILM FORMATION A N D POTENTIAL MEASUREMENTS OF O.-BRASS SPECIMENS IN SOLUTIONS OF
0'88 tool/1 OF AMMO~A + 0"05tool/1 OF COPPER WITH pH 7-12"0 Original potential pH
Black
7.0 7.1 7'2 7"3 7.4 7'5 7'6 7.7 7.8 7"9 8.1 8.2 8'6 9.0 9.5 10"0 10"2 10.5 10.7 10"8 11"0 11"2 11"4 11"6 11'8 12.0
40 rain
30 rain 20 nun 20 mm 30 rain 40 min 40 mm 5O mill 50 nun 60 rain more than 10 h 10h 10 h 10h 10h 10h 10h 10 h 10h 9h 7h 5h 3h 2h 1½h 45 rain
(mV) E ~
Changein first hour
(mV)
224 215
+ 112 + 108
190
+ 40
162 150 118
+ 22 + 17
100
0
+
4
-20 -- 104
--
20 0
--
110
+
2
---
121 119
+ +
3 16
-- 113 -- 110 -- 107
+ + +
17 17 23
93
+ 60
Stress corrosion crack propagation in u-brass in ammoniacal solutions
627
Tarnish f i l m examination
The tarnish film was identified by X-ray diffraction of a powder sample. Specimens were pickled in HNOa and then immersed in the tarnishing solution for 24 h. After this time the tarnish layer was scraped off. The solutions employed and the results obtained are shown in Table 4. In solutions 2-5 all the patterns revealed Cu20 while in solution 1 ZnO and ZnSO4.3Zn(OH)2 were also detected. Cu20 has been observed and identified by many workers but not the other compounds. For example, Cu20 has been identified on brass exposed to Mattsson solutions at pH 2 and 10, 2o and at 3.8, 5.6 and 7.2. 9 Potential measurements
To determine the effect of the tarnish film on the rest potential the potential of specimens was measured as a function of time. The results are shown in Table 3 in which the comparison can be made with the tarnishing rates already described. In the neutral range of pH a potential rise occurred in tarnishing solutions which can be interpreted as indicating the relative protective nature of the film, whereas in nontarnishing solutions no appreciable rise in potential occurred. These results are in agreement with those of Hoar et al. 9,1° but somewhat more extensive. The rate of tarnish film formation appeared to be related to the rate of potential rise, both being at their highest at pH 7.2 and 7.3. In the alkaline range of pH the situation was different. At pH 12.5 the potential did not move from the original value of -- 234 mV (v. S.C.E.) even though the specimen was tarnishing slowly. This observatio~a is in agreement with the work of Green et al. 2~ with 15N ammonia which causes rapid tarnishing. They observed that the applied current required to sustain a given potential during film growth was constant and concluded that the film forming in these solutions is porous and non-protective. DISCUSSION
The highest rates of propagation in the less concentrated solution were observed at pH values near to the borderline conditions for the formation of both soluble species and insoluble tarnish film simultaneously. Although the structure of the film formed at neutral pH is not known, it grows parabolically as and it causes a rise in potential. It affords some degree of protection to the alloy. It appears therefore that a relatively simple mechanism is operative. Preferential anodic dissolution occurs at the crack tip and causes a maximum crack velocity when tarnishing occurs sufficiently rapidly to cover the sides of the crack and thus concentrate the dissolution at the crack tip. Any striations on the grain boundary surface arise from plastic deformation either during or immediately after the passage of the crack tip and would be rapidly tarnished. Such a cracking process would become slow when tarnishing occurred at a rate sufficient to cover the crack, or if dissolution was occurring on a larger area due to too slow a tarnishing rate so that the same total current resulted in a slower rate of corrosion at the crack tip. It appears that a similar mechanism of preferential anodic dissolution occurred in the stronger ammoniacal solution over the pH range 10-13.5. In these solutions the rate of crack propagation increased with increasing tarnishing rate. Little work has been done on the morphology of films forming in either of the two pH ranges but the evidence of Table 3 suggests that the two tarnish films have different properties. In the
G. M. SPARKESand J. C. SCULLY
628 TABLE 4.
IDENTIFICATION OF TARNISH FILMS FORMED ON BRASS IN FIVE SOLUTIONS OVER 24 h
Initial pH (1) (2) (3) (4) (5)
0.88 mol/l of ammonia + 0.05 mol/l of copper 0.88 mol/l of ammonia + 0.05 mol/l of copper 0"88 mol/l of ammonia + 0"05 mol/l of copper 7"8 mol/l of ammonia + 0.05 mol/l of copper 7.8 mol/l of ammonia + 0.05 mol/l of copper d values
CusO
8.14 10"83 11'2 12.5 13.5
ZnSO,.3Zn(OH)24HzO
Final pH 8.17 11"06 11"22
12.6 13"7 ZnO
Solution (1) pH 8'14
9"96(100) 5"80 (20) 5.15 (30) 4.17 (30) 3.85 (10) 3.45 (15) 3.22 (40) 3"11 (30) 3.01 (30) 2.72 (40) 2.60 (45) 2.41(100) 2.11(100) 1.82 (20) 1.65 (10) 1.56 (20) 1.49 (30) 1"29 (30)
10"0 (100) 5"91 (11) 5.11 (9) 4.19 (40) 3.83 (9) 3.61 (3) 3.28 (11) 3"I1 (9) 3.02 (9) 2.75 (90) 2.65 (35) 2.46(100) 2.13 (37)
2.09 (10) 1.89 (3) 1.62 (40) 1-56 (13)
1.51 (27) 1"28 (17)
Sohaion (2) pH 10.83
2.90 (30) 2.43(100) 2.11 (80) 1.51 (70) 1.27 (40) 1.25 (10)
3.02 (9) 2.46(100) 2.13 (37) 1.51 (27) 1-28 (17) 1.23 (4)
Solution (3) pH 11"2
3"03 (20) 2"47(100) 2"13 (80) 1.50 (70) 1.29 (20)
3.02 (9) 2.46(100) 2'13 (37) 1.51 (27) 1.28 (17)
Solution (4) pH 12.5
2.99 (25) 2.44(100) 2.12 (60) 1.50 (50)
3.02 (9) 2.46(100) 2.13 (37) 1.51 (27)
Solution (5) pH 13"5
2"98 (10) 2.45(100) 2"11 (80) 1"49 (50) 1.27 (20)
2.60 (56) 2.47(100)
3.02 (9) 2.46(100) 2"13 (37) 1.51 (27) 1.28 (17)
Figures in brackets refer to relative intensities.
1.47 (35)
Stress corrosion crack propagation in a-brass in ammoniacal solutions
629
alkaline solution the film is non-protective. The growth rate is linear 21 and the preferential corrosion of grain boundaries to considerable depth in unstressed specimens 3 in such solutions can be taken as evidence for the porous and, general nonprotective characteristics of such tarnish films. Intergranular corrosion and cracking have been explained 3 as arising from the segregation of zinc to grain boundary regions. Transgranular cracking has been attributed 22 to the reactivity of dislocation pile-ups where some solute segregation may have occurred. The possible role of zinc is to dissolve preferentially giving rise to a high density of copper kink sites. 23 The path that the crack takes will be determined by these considerations and also by the properties of the film. Where there is either no tarnishing, very slow tarnishing or the formation of porous non-protective tarnish films, there will be two possible paths for the crack to follow with the preference being dependent upon the relative reactivity of the two zinc-rich regions. The intergranular path is pre-existing, evidence for which is shown by the grain boundary penetration of non-stressed specimens, s whereas the transgranular path is brought into existence as an effect of the applied stress. The intergranular one is likely to be preferred, particularly at low plastic strain where the density of dislocations will be low and where protective tarnish films form on grain surfaces, although as Table 1 indicates, such influences are not complete. Increasing amounts of plastic deformation will tend to break up grain boundary regions and might be expected to increase the amount of transgranular cracking but no such changes were observed in the cold-worked bar material nor in the annealed sheet. A transition to transgranular mode can be expected only when the reactivity of transgranular features is comparable to that of grain boundary material. Even when this "comparable reactivity" is established the grain boundaries will continue to remain highly reactive and any anodic current will be distributed between these and the transgranular features. From these rather simple considerations of a complicated process it could be speculated that a high corrosion rate may be important in determining the transition from intergranular to transgranular cracking in addition to the absence of a protective tarnish film. It is a characteristic of transgranular cracking that it is accompanied by evidence of noticeable increased rates of corrosion, e.g. wide cracks with many small branches. 9 Consistent with this is the observation that intergranular to transgranular transition is promoted by anodic polarization and reduced by cathodic polarization. 6 Transgranular cracking occurs at pH values of pH 13.5 for which the weight loss is higher a than for any of the solutions reported in Table 2. Evidence of this work is that intergranular cracking is the dominant form of fracture except at very high pH and that the tarnish film serves to suppress the occurrence of transgranular cracking when it affords some degree of protection to the surface on which it forms. In neutral solutions the tip of the crack surface is yellow and there is no need therefore to involve the fracture of a protective black film. In alkaline solutions the unprotective nature of the black film also makes such fracture unimportant and therefore unnecessary. Mattsson 4 reported a simple distinction between the fracture effects of tarnishing and non-tarnishing solutions. The former caused intergranular fracture while the latter caused transgranular fracture. Such ideas have been supported by other workers, s,5 It is not clear where the difference in observed effects can be explained.
630
G. M. SPARKESand J. C. SCULLY
The cold w o r k in the b a r c a n n o t be involved since similar fractures and results were observed on the experiments in neutral a n d highly alkaline solutions on the annealed sheet. T h e fractures observed on brass are d e p e n d e n t u p o n the degree o f cold work, 9 potential s a n d c o m p o s i t i o n , b o t h o f the brass a n d o f the solution. I n the m a t e r i a l examined the fracture was mainly i n t e r g r a n u l a r and the f o r m a t i o n o f a tarnish film was not associated with the exclusive occurrence o f i n t e r g r a n u l a r cracking since while a high p r o p o r t i o n o f such cracking occurred in neutral solutions, in the high alkaline tarnishing solutions cracking was mainly transgranular. CONCLUSION Since no exclusive form o f fracture has been observed for any value o f p H within the wide range that has been studied, it is suggested t h a t p e r h a p s only one m e c h a n i s m o f fracture is operative in which localized dissolution occurs. This is t h o u g h t to arise from local concentrations o f zinc a t o m s at grain b o u n d a r i e s or in dislocations a l t h o u g h evidence for this has n o t been provided. T h e specific cause o f the variations in the average velocity has n o t been elucidated a n d would a p p e a r to d e p e n d u p o n a closer study o f velocity as a function o f stress intensity factor, potential, p H a n d a m m o n i a a n d dissolved c o p p e r content. This w o r k is now in progress 2~ and will be r e p o r t e d later.
Acknowledgements--One of the authors (G.M.S.) is grateful to the Science Research Council for a studentship for the whole of the period in which this work was carried out.
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