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Localized corrosion at welds in structural steel under desalination plant conditions Part II: Effect of heat treatment, test temperature and test media
407
Desalination,73 (1989)407-415
Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
Localized corrosion at welds in structural steel under desalination plant conditions Part II: Effect of heat treatment, test temperature and test media NABIL M.A. EID Associate Professor, Petroleum & Gas Technology Division, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia
In a previous instalment, the effect of surface roughness and type of welding electrode on localized corrosion at welds in structural steel under desalination plant conditions have been discussed. The present part deals with the effect of heat treatment, test temperature and test media.
1. HEAT TREXl-MENT
IN RELATION TO CORROSION PROPERTIES
As shown in the previous section, the state of aggregation of the metal, the crystal size, and the presence of different phases in the metal and their distribution, are the main factors controlling corrosion. Between the weld metal, which reaches its melting point and above, and the cold parent metal, there are zones within which all intermediate temperatures are reached promoting localized changes in the microstructure. Heat treatment is an operation involving the heating of the solid metal to definite temperatures, followed by cooling at suitable rates in order to obtain certain changes in the nature, form, size and distribution of the micro-constituents; in what follows are some of the heat treatment operations that are commonly used. Annealing
The purpose of annealing may involve one or more of the following aims: 1. To soften the steel; improve machinability 2. To relive internal stresses induced by some previous treatment 3. To remove coarseness of grain. The operation consists in heating the steel to a certain temperature; soaking at this temperature for a time sufficient to allow the necessary changes to occur, then cooling at a predetermined rate. It is not always necessary to heat the steel into the critical range
408
ten..,
,
I.,-.
_.
__
800
P
&
ci+fe,C
,
1
639 I
G
I
I
-
-600
A 0
65
I
2 Carbon, Cementite
Figure 1:
Fe-Fe&
equilibrium diagram
per
3 cent
+ Pear&e-
4
5
and the mild steel products are annealed by heating at 500 to 650°C for several hours; this method is commonly employed for sheets. Generally, the recrystallization temperature of pure iron is in the region of 5OO”C,consequently the higher temperature of 650°C brings about rapid recrystallization of the distorted ferrite, and since mild steel contains only a small volume of strained pearlite a high degree of softening is induced. Normalizing
treatment
For steels with less than 0.9% carbon, the treatment consists in heating the steel to about 935-970°C; i.e., slightly above the upper critical point indicated by the Fe-Fe C equilibrium diagram, see Fig. 1. After soaking at the temperature for a time dependent on the thickness of the specimen, the metal is allowed to cool in still air. The structure produced, however, varies with the thickness of the metal treated. Quench
hardening
of steel
Hardening of steel is obtained by a suitable quench from within or above the critical range. The temperatures are the same as those given in normalizing treatment. The soaking time in air furnaces should be 1.2 mm/mm of cross-section. Uneven heating, overheating, and excessive scaling should be avoided. The quenching is necessary to suppress the normal breakdown of austenite into ferrite and cementite, and to cause a partial decomposition at such a low temperature that martensite is produced. Steels with less than 0.3% carbon cannot be hardened effectively, while the maximum effect is obtained at about 0.7% due to an increased tendency to retain austenite in high carbon steels. Water is one of the most efficient quenching media in commercial use where maximum hardness is required, but it is liable to cause distortion and cracking of the article. Oils are also used for quenching; these tend to oxidise and form sludge with consequent lowering of efficiency. The quenching velocity of oil is much less than water; ferrite and troostite are formed even in small sections. In the present work, ample evidence is available showing improvement of corrosion characteristics of heat treated specimen over the untreated ones. The results are encouraging and promising in the battle against corrosion. However, in order to be in a position to draw solid conclusions and to build sound rules on this aspect of the problem, further experiments are needed.
2. EFFECT OF TEMPERATURE
The results show an accelerated effect on the corrosion rate. Generally, when corrosion is controlled by diffusion of oxygen, the corrosion rate at a given oxygen concentration approximately doubles for every 30°C (55°F) rise in temperature. In an open vessel, allowing dissolved oxygen to escape, the rate increases with temperature reaching its maximum at 60°C (175V), then falls to a very low rate value at the boiling point; see Fig. 4. The falling off of corrosion rate is related to the marked decrease of oxygen solubility in water as the temperature is raised. This effect
410
Ttmperslurc.
Figure 2:
lC
Effect of temperature on corrosion of iron in water containing dissolved oxygen
eventually overshadows the accelerating effect of temperature alone. On the other hand, in a closed system, oxygen cannot escape and the corrosion rate continues to increase with temperature until all the oxygen is consumed. When corrosion is attended by hydrogen evolution, the rate of increase will increase - more than double for every 30°C (55’F) rise in temperature. A notable feature in the present experiments on stainless steel/stainless weldments is the formation of a deposited protective coating on surface of specimens tested at 25°C; for further information see section on effect of welding electrode.
3. EFFECT OF TESTING MEDIA
The most common corroding agent used is 3.5% sodium chloride solution, though many other solutions have been used, including distilled water, ammonia, hydrochloric acid, sulphuric acid, sodium nitrate, ammonium chloride and potassium chloride. Most of the work using these simple solutions has been carried out with the object of investigating various aspects of the mechanism of corrosion. Thus, in fundamental research the media employed may be chosen for particular theoretical reasons, while practical tests will often be made in the medium peculiar to the service conditions, or in a laboratory approximation to that medium. In the present research, a comparison study was made in synthetic and natural seawaters. Careful inspection of the results reveals the appreciable difference in the results of the two corrosive media (refs. 1 and 2). At this juncture, it is of importance to point out that stainless steel gave very good results in synthetic seawater, but in natural seawater, pitting corrosion occurred - pitting is essentially the only form of attack. Generally speaking, in the metabolism of any living organism, there is energy intake and an energy release. In some bacteria these energy changes result in a
411
measurable electric current. When corrosion - causing organisms reproduce on the surface of an iron surface, electrobiochemical removal of iron takes place. Microorganisms then, can participate in iron and steel corrosion by creating favorable conditions for electrochemical reaction to take place. One mechanism is the changing of a surface film resistance with metabolic products such a sulphuric or organic acids. In other instances slime deposits can form in selected areas so that they become anaerobic. If oxygen is present in solution, the anaerobic sites become anodic to the aerobic areas. When two portions of the same metal receive oxygen at different rates, a corrosion cell is established and two types of oxidation can take place - dehydration and loss of electrons. The hydrogen atom dissociates into a proton and an electron. Electrons are transported by an electron-carrying system and pass through the corroding metal to the cathodes; where each electron displaces a positive ion, usually hydrogen. The positive metal ions formed at the anodes go into solution. In brief, no laboratory approximation can replace site testing when the media and conditions are complex, as in desalination plant conditions. The relevant factors are extremely difficult to reproduce, since they may include extraneous deposits (algae, crustaceous, etc.) or even decomposing marine organisms due to which the attack at the stainless steel is referred to.
4. EFFEXTOFRELATIVEVELDCfIYOFTHESEAWATER
The relative velocity of the seawater has a major influence on the corrosion behavior of stainless steels; the various deposits that constitute so great a danger cannot form so readily when the water is in motion, while compared with stagnant water the oxygen supply that maintains passivity remains more constant. Thus, it has been shown in Table 1 that water pumped at a velocity of 1.5 m/set is much less corrosive than stagnant water (ref. 3). Table 1: Corrosion resistance of steels in seawater (Influence of water movement on 18/8 steel immersed for 13 months; depth of pits in inches) Unwelded Plate
Welded Plate
-----~-------________ Stagnant water
pumped water
Stagnant water
Pumped water
0.039 0.104
0.000 0.004
0.291 0.330 0.153 0.193
0.000 0.000 0.000 0.000
412
CONCLUSIONS
Research into the corrosion resistance of 316L stainless steel in seawater, has shown that it is highly sensitive to adverse factors such as temperature and the combined action of air and deposits of salt crystals. However, the stainless steel under consideration (M/~/MO grade), in contrast with the mild steel, it can be considered quite resistant to seawater and can be used at the desalination plant conditions, providing soaking the steel in the seawater at 25°C for two weeks (or more) until a protective layer (probably oxide) is formed on the surface of the steel. Mechanisms
of
corrosion
Table 2 is showing metals in order of increasing potential in seawater. It should be noticed that the steel under consideration (i.e., 18/8/Mo grade) and its derivatives are listed twice at different points; in the active state, which occurs for instance in the bottom of a corrosion pit, they are placed between iron and lead, whereas in the passive state they are the most noble of the metals and alloys in seawater. When in contact with any metal higher up in the list, they are protected at the expense of the accelerated corrosion of the mild steel. Table 2: The galvanic series for metals used in seawater (after Uhlig) Magnesium Magnesium allows zinc Galvanized iron and steel Aluminium and duralumin Mild steel Wrought iron Cast iron 13% Cr stainless steel (active) Solder (50% Pb, 50% Sn) 18/8 stainless steel (active) Lead Tin Muntz metal Manganese bronze Naval brass Nickel (active) Uhlig, H.H. The Corrosion
Handbook, Chapman,
Nichrome (active) Yellow brass Admiralty bronze Aluminium bronze Red brass Copper Silicon bronze Nickel silver 70130 cupronickel 88/10 bronze with 2% Zn 88/6 l/2 bronze with 3% Zn and 1 l/2% Pb Nickel (passive) Nichrome (passive) Monel metal 18/8 stainless steel (passive) 18/8/Mo stainless steel (passive)
New York, 1948.
This can be applied equally to a cell consisting of a stainless steel in the passive state and the same steel in the active state (inside the corrosion pits). The large surface area of unattacked stainless steel results in very rapid attack at the points where pitting is initiated. In case of mild steel/stainless steel welded specimens, the mild steel will protect the stainless steel. Its action is highly efficient, and with this protection 18/8/Mo stainless steel will stand up very well to the worst conditions (i.e., desalination plant conditions).
413
The surface area of the mild steel must be similar to or greater than that of stainless steel which is to be protected, since if its area is disproportionately small it will corrode away so rapidly as to be useless. Similar reasons underlie the high resistance to seawater corrosion of weld metal in mild steel specimens. The welds are completely resistant, but because of their small surface area compared with the parent metal, the corrosion on the latter is not increased appreciably. On the basis of numerous experiments and practical observations, Laque (ref. 4) drew up a summarized chart, see Fig. 6, which gives a good general picture of the behavior of stainless steels in contact with other metals in seawater. The chart is invaluable as a guide to selection. Welding
instructions
The welding must be carried out with extreme care to avoid porosity and oxide inclusions, either of which can provoke pitting corrosion. Generally speaking, welding is very widely applied to stainless steels, and all the processes available for ordinary steels can be used, with sole exception of forge welding. This is rendered impossible by the formation of chromium-rich scale layers. All the remaining processes, including arc and gas welding, spot welding and flash-butt welding, can be used provided the conditions are adjusted to the specific properties of the steel concerned (refs. 5-12). In the welding of austenitic steels however, two dangers are to be borne in mind: 1. The heat-affected zone round the weld may become sensitive to intergranular corrosion. 2. Welded joints are liable to hot cracking under the stresses set up during cooling, because of the large thermal contraction; the danger increases with increasing constraint on the joint. The dominant factor in the welding of austenitic steels therefore is that the heataffected zones surrounding the weld harden on cooling, setting up stresses which can give rise to cracking. If the filler metal is similar to the parent metal, the weld itself will also harden on cooling and become highly brittle. Two precautions are needed to avoid trouble from Urisdirection: 1. Austenite filler metal should be used; it remains ductile on cooling and can absorb the stresses set up by hardening in the heat affected zone without cracking. 2. The work is preheated gradually before welding, and reheated throughout at the end of the work to avoid quenching stresses and cracks. How to avoid intergranular
corrosion
and thermal cracking at the weld?
Between the weld metal, which reaches its melting point and above, and the cold parent metal, there is a zone within which all intermediate temperatures are reached, including points at a certain distance from the weld junction where the temperature promotes carbide precipitation. Consequently, a sensitized zone runs parallel to the weld on either side.
414
Certain grades of austenitic steel are immune to intergranular corrosion under all conditions of heat treatment, some because of their very low carbon content, other because their structure is duplex and other again by virtue of stabilizing additions of Ti or Nb. Trouble must be anticipated with unstabilized grades, and heat treatment is essential to make them resistant in certain corrosive media. If full heat treatment is out of the question, because of size or the risk of distortion, it may be sufficient in some cases to temper for 4 hours at 850°C followed by air cooling. This reduces the risk of intergranular corrosion but is by no means as efficient as full solution treatment (ref. 13). At this juncture it might be of interest to point out that the low carbon and stabilized grades of steel are now used on a very wide scale, and post-heat treatment is seldom carried out. It should be noted however, that, the risk of intergranular corrosion is not confined to the heat affected zones near the welds. A single cycle of fusion followed by solidification is usually insufficient to sensitize the weld metal to the same extent as other cycles in which the maximum temperature is below the melting point. Nevertheless, local sensitization will almost inevitably occur if a weld is reheated, as when other deposits are made adjacent to it. These conditions are encountered for instance at weld intersections; again, when a circumferential joint is made and the welder arrives back at the starting point, which by then will have cooled down, local sensitization will occur. In other words, the filler metal must itself be of a stabilized grade. The choice lies between very low carbon and Nb stabilized steels-titanium is useless because it oxidizes and the weld deposit will not contain sufficient for stabilization. The best solution, however, is to select a filler metal with a balanced composition that results in a duplex austenite-ferrite weld meal structure. Duplex structures must on the other hand be avoided if intergranular corrosion is not the only consideration. Corrosion resistance in certain media, e.g., boiling nitric acid solutions at concentrations in excess of 20% I-IN* is contingent upon a fully austenitic structure in both parent and weld metals. This limits the choice to austenitic filler metals with very low carbon contents or Nb stabilization (in addition to bare wire for argon-shielded arc welding, coated electrodes are now available that deposit weld metal with less than 0.02% C). Great care must also be taken to avoid weld cracking which would be initiated at high temperatures, after the metal has solidified. Generally, crack sensitivity is a function of the weld metal composition. Experience has shown that welds with a duplex austenite-ferrite structure are immune to cracking. Ferrite is more ductile at very high temperatures and the metal consequently accommodates itself more easily to the stresses that are usually at the origin of the cracking. The most popular types of duplex electrode contain 0.03-0.06% C, 18-21% Cr. 8-10% Ni and 2-3% MO, the steel is known as 20-10-3. Attempts are sometimes made to control the ferrite content of the weld metal within a certain range; the minimum corresponds to good crack resistance at a specified severity of constraint and the maximum to specified mechanical properties or corrosion resistance. The range commonly adopted is 3 to 6%.
415
So far we have been discussing cracking in the weld metal itself. Occasionally, though far less frequently, cracking may be observed in the parent metal immediately alongside the fusion boundary. Cracking may still occur if the well metal is a crackresistant duplex type. The trouble is quite specific to steels containing niobium and does not occur in the Ti-stabilized grades. It is also known that cracks can propagate through the heat-affected zones if the weldment is reheated; in other words, these cracks can occur during stress-relieving heat treatments (550-950°C) or after some service at elevated temperatures. For further information, the reader is referred to a number of references to the welding of stainless steels (refs. 5-16).
REFERENCES 1.
Eid, N.M.A. et al. SPE Technical 4, 1989.
2.
Eid, N.M.A. 4th Middle East Corrosion Conference, 11-13 January, 1988. Bahrain, pp. 585-819. Symposium on Advances on Oil Field Chemistry, Third Chemical Congress of the Northern American Continent and 195th national Meeting of the American Chemical Society, Toronto, Canada, June 5-11, 1988.
3. 4. 5. 6. 7. 8. 9.
Larrabee,
Aramco,
C.P. Trans. Electra Chem. pp. 161-182.
Kreischer,
C.H. et al. Welding
Henry, O.H. and Claussen. Younger,
Dhahran, Saudi Arabia, April
1945.
M. In Steels for Reactor Pressure Circuits, ISI, 1960.
Laque, F.L. and Cardouni,
Myers,
Symposium;
J., 40, p. 489, 1961.
G.E. Welding
Metallurgy.
p. 392, 1949.
R.N. and Baker, R.G. J151, 194. p. 189, 1960.
J. J. Brit. Welding,
Robertson,
9, p. 677. 1962.
J.M. J. Brit. Welding,
10.
Truman,
R.Y. J. Brit. Welding,
11.
Christoffel,
12.
Evans, N.C. J151, 149, p.
R.J.
J. Welding,
8. p. 643. 1961.
9, p. 317, 1962. 41, p. 251. 1962.
275, 1944.
13.
Houndremont,
14.
Williams,
N.T. and Willoughby,
E. Handhuch der Somers, Lahlkunde. 2nd Ed. p. 554.
15.
Emerson,
R.W. Welding
16.
Roques, C. and Guegnaud.
G. Brit. Welding
J. 9, p. 115, 1962.
J. 41, p. 383, 1962. A. Rev. Met. 61, p. 505, 1964.
Desalination,73 (1989)407-415
Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
Localized corrosion at welds in structural steel under desalination plant conditions Part II: Effect of heat treatment, test temperature and test media NABIL M.A. EID Associate Professor, Petroleum & Gas Technology Division, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia
In a previous instalment, the effect of surface roughness and type of welding electrode on localized corrosion at welds in structural steel under desalination plant conditions have been discussed. The present part deals with the effect of heat treatment, test temperature and test media.
1. HEAT TREXl-MENT
IN RELATION TO CORROSION PROPERTIES
As shown in the previous section, the state of aggregation of the metal, the crystal size, and the presence of different phases in the metal and their distribution, are the main factors controlling corrosion. Between the weld metal, which reaches its melting point and above, and the cold parent metal, there are zones within which all intermediate temperatures are reached promoting localized changes in the microstructure. Heat treatment is an operation involving the heating of the solid metal to definite temperatures, followed by cooling at suitable rates in order to obtain certain changes in the nature, form, size and distribution of the micro-constituents; in what follows are some of the heat treatment operations that are commonly used. Annealing
The purpose of annealing may involve one or more of the following aims: 1. To soften the steel; improve machinability 2. To relive internal stresses induced by some previous treatment 3. To remove coarseness of grain. The operation consists in heating the steel to a certain temperature; soaking at this temperature for a time sufficient to allow the necessary changes to occur, then cooling at a predetermined rate. It is not always necessary to heat the steel into the critical range
408
ten..,
,
I.,-.
_.
__
800
P
&
ci+fe,C
,
1
639 I
G
I
I
-
-600
A 0
65
I
2 Carbon, Cementite
Figure 1:
Fe-Fe&
equilibrium diagram
per
3 cent
+ Pear&e-
4
5
and the mild steel products are annealed by heating at 500 to 650°C for several hours; this method is commonly employed for sheets. Generally, the recrystallization temperature of pure iron is in the region of 5OO”C,consequently the higher temperature of 650°C brings about rapid recrystallization of the distorted ferrite, and since mild steel contains only a small volume of strained pearlite a high degree of softening is induced. Normalizing
treatment
For steels with less than 0.9% carbon, the treatment consists in heating the steel to about 935-970°C; i.e., slightly above the upper critical point indicated by the Fe-Fe C equilibrium diagram, see Fig. 1. After soaking at the temperature for a time dependent on the thickness of the specimen, the metal is allowed to cool in still air. The structure produced, however, varies with the thickness of the metal treated. Quench
hardening
of steel
Hardening of steel is obtained by a suitable quench from within or above the critical range. The temperatures are the same as those given in normalizing treatment. The soaking time in air furnaces should be 1.2 mm/mm of cross-section. Uneven heating, overheating, and excessive scaling should be avoided. The quenching is necessary to suppress the normal breakdown of austenite into ferrite and cementite, and to cause a partial decomposition at such a low temperature that martensite is produced. Steels with less than 0.3% carbon cannot be hardened effectively, while the maximum effect is obtained at about 0.7% due to an increased tendency to retain austenite in high carbon steels. Water is one of the most efficient quenching media in commercial use where maximum hardness is required, but it is liable to cause distortion and cracking of the article. Oils are also used for quenching; these tend to oxidise and form sludge with consequent lowering of efficiency. The quenching velocity of oil is much less than water; ferrite and troostite are formed even in small sections. In the present work, ample evidence is available showing improvement of corrosion characteristics of heat treated specimen over the untreated ones. The results are encouraging and promising in the battle against corrosion. However, in order to be in a position to draw solid conclusions and to build sound rules on this aspect of the problem, further experiments are needed.
2. EFFECT OF TEMPERATURE
The results show an accelerated effect on the corrosion rate. Generally, when corrosion is controlled by diffusion of oxygen, the corrosion rate at a given oxygen concentration approximately doubles for every 30°C (55°F) rise in temperature. In an open vessel, allowing dissolved oxygen to escape, the rate increases with temperature reaching its maximum at 60°C (175V), then falls to a very low rate value at the boiling point; see Fig. 4. The falling off of corrosion rate is related to the marked decrease of oxygen solubility in water as the temperature is raised. This effect
410
Ttmperslurc.
Figure 2:
lC
Effect of temperature on corrosion of iron in water containing dissolved oxygen
eventually overshadows the accelerating effect of temperature alone. On the other hand, in a closed system, oxygen cannot escape and the corrosion rate continues to increase with temperature until all the oxygen is consumed. When corrosion is attended by hydrogen evolution, the rate of increase will increase - more than double for every 30°C (55’F) rise in temperature. A notable feature in the present experiments on stainless steel/stainless weldments is the formation of a deposited protective coating on surface of specimens tested at 25°C; for further information see section on effect of welding electrode.
3. EFFECT OF TESTING MEDIA
The most common corroding agent used is 3.5% sodium chloride solution, though many other solutions have been used, including distilled water, ammonia, hydrochloric acid, sulphuric acid, sodium nitrate, ammonium chloride and potassium chloride. Most of the work using these simple solutions has been carried out with the object of investigating various aspects of the mechanism of corrosion. Thus, in fundamental research the media employed may be chosen for particular theoretical reasons, while practical tests will often be made in the medium peculiar to the service conditions, or in a laboratory approximation to that medium. In the present research, a comparison study was made in synthetic and natural seawaters. Careful inspection of the results reveals the appreciable difference in the results of the two corrosive media (refs. 1 and 2). At this juncture, it is of importance to point out that stainless steel gave very good results in synthetic seawater, but in natural seawater, pitting corrosion occurred - pitting is essentially the only form of attack. Generally speaking, in the metabolism of any living organism, there is energy intake and an energy release. In some bacteria these energy changes result in a
411
measurable electric current. When corrosion - causing organisms reproduce on the surface of an iron surface, electrobiochemical removal of iron takes place. Microorganisms then, can participate in iron and steel corrosion by creating favorable conditions for electrochemical reaction to take place. One mechanism is the changing of a surface film resistance with metabolic products such a sulphuric or organic acids. In other instances slime deposits can form in selected areas so that they become anaerobic. If oxygen is present in solution, the anaerobic sites become anodic to the aerobic areas. When two portions of the same metal receive oxygen at different rates, a corrosion cell is established and two types of oxidation can take place - dehydration and loss of electrons. The hydrogen atom dissociates into a proton and an electron. Electrons are transported by an electron-carrying system and pass through the corroding metal to the cathodes; where each electron displaces a positive ion, usually hydrogen. The positive metal ions formed at the anodes go into solution. In brief, no laboratory approximation can replace site testing when the media and conditions are complex, as in desalination plant conditions. The relevant factors are extremely difficult to reproduce, since they may include extraneous deposits (algae, crustaceous, etc.) or even decomposing marine organisms due to which the attack at the stainless steel is referred to.
4. EFFEXTOFRELATIVEVELDCfIYOFTHESEAWATER
The relative velocity of the seawater has a major influence on the corrosion behavior of stainless steels; the various deposits that constitute so great a danger cannot form so readily when the water is in motion, while compared with stagnant water the oxygen supply that maintains passivity remains more constant. Thus, it has been shown in Table 1 that water pumped at a velocity of 1.5 m/set is much less corrosive than stagnant water (ref. 3). Table 1: Corrosion resistance of steels in seawater (Influence of water movement on 18/8 steel immersed for 13 months; depth of pits in inches) Unwelded Plate
Welded Plate
-----~-------________ Stagnant water
pumped water
Stagnant water
Pumped water
0.039 0.104
0.000 0.004
0.291 0.330 0.153 0.193
0.000 0.000 0.000 0.000
412
CONCLUSIONS
Research into the corrosion resistance of 316L stainless steel in seawater, has shown that it is highly sensitive to adverse factors such as temperature and the combined action of air and deposits of salt crystals. However, the stainless steel under consideration (M/~/MO grade), in contrast with the mild steel, it can be considered quite resistant to seawater and can be used at the desalination plant conditions, providing soaking the steel in the seawater at 25°C for two weeks (or more) until a protective layer (probably oxide) is formed on the surface of the steel. Mechanisms
of
corrosion
Table 2 is showing metals in order of increasing potential in seawater. It should be noticed that the steel under consideration (i.e., 18/8/Mo grade) and its derivatives are listed twice at different points; in the active state, which occurs for instance in the bottom of a corrosion pit, they are placed between iron and lead, whereas in the passive state they are the most noble of the metals and alloys in seawater. When in contact with any metal higher up in the list, they are protected at the expense of the accelerated corrosion of the mild steel. Table 2: The galvanic series for metals used in seawater (after Uhlig) Magnesium Magnesium allows zinc Galvanized iron and steel Aluminium and duralumin Mild steel Wrought iron Cast iron 13% Cr stainless steel (active) Solder (50% Pb, 50% Sn) 18/8 stainless steel (active) Lead Tin Muntz metal Manganese bronze Naval brass Nickel (active) Uhlig, H.H. The Corrosion
Handbook, Chapman,
Nichrome (active) Yellow brass Admiralty bronze Aluminium bronze Red brass Copper Silicon bronze Nickel silver 70130 cupronickel 88/10 bronze with 2% Zn 88/6 l/2 bronze with 3% Zn and 1 l/2% Pb Nickel (passive) Nichrome (passive) Monel metal 18/8 stainless steel (passive) 18/8/Mo stainless steel (passive)
New York, 1948.
This can be applied equally to a cell consisting of a stainless steel in the passive state and the same steel in the active state (inside the corrosion pits). The large surface area of unattacked stainless steel results in very rapid attack at the points where pitting is initiated. In case of mild steel/stainless steel welded specimens, the mild steel will protect the stainless steel. Its action is highly efficient, and with this protection 18/8/Mo stainless steel will stand up very well to the worst conditions (i.e., desalination plant conditions).
413
The surface area of the mild steel must be similar to or greater than that of stainless steel which is to be protected, since if its area is disproportionately small it will corrode away so rapidly as to be useless. Similar reasons underlie the high resistance to seawater corrosion of weld metal in mild steel specimens. The welds are completely resistant, but because of their small surface area compared with the parent metal, the corrosion on the latter is not increased appreciably. On the basis of numerous experiments and practical observations, Laque (ref. 4) drew up a summarized chart, see Fig. 6, which gives a good general picture of the behavior of stainless steels in contact with other metals in seawater. The chart is invaluable as a guide to selection. Welding
instructions
The welding must be carried out with extreme care to avoid porosity and oxide inclusions, either of which can provoke pitting corrosion. Generally speaking, welding is very widely applied to stainless steels, and all the processes available for ordinary steels can be used, with sole exception of forge welding. This is rendered impossible by the formation of chromium-rich scale layers. All the remaining processes, including arc and gas welding, spot welding and flash-butt welding, can be used provided the conditions are adjusted to the specific properties of the steel concerned (refs. 5-12). In the welding of austenitic steels however, two dangers are to be borne in mind: 1. The heat-affected zone round the weld may become sensitive to intergranular corrosion. 2. Welded joints are liable to hot cracking under the stresses set up during cooling, because of the large thermal contraction; the danger increases with increasing constraint on the joint. The dominant factor in the welding of austenitic steels therefore is that the heataffected zones surrounding the weld harden on cooling, setting up stresses which can give rise to cracking. If the filler metal is similar to the parent metal, the weld itself will also harden on cooling and become highly brittle. Two precautions are needed to avoid trouble from Urisdirection: 1. Austenite filler metal should be used; it remains ductile on cooling and can absorb the stresses set up by hardening in the heat affected zone without cracking. 2. The work is preheated gradually before welding, and reheated throughout at the end of the work to avoid quenching stresses and cracks. How to avoid intergranular
corrosion
and thermal cracking at the weld?
Between the weld metal, which reaches its melting point and above, and the cold parent metal, there is a zone within which all intermediate temperatures are reached, including points at a certain distance from the weld junction where the temperature promotes carbide precipitation. Consequently, a sensitized zone runs parallel to the weld on either side.
414
Certain grades of austenitic steel are immune to intergranular corrosion under all conditions of heat treatment, some because of their very low carbon content, other because their structure is duplex and other again by virtue of stabilizing additions of Ti or Nb. Trouble must be anticipated with unstabilized grades, and heat treatment is essential to make them resistant in certain corrosive media. If full heat treatment is out of the question, because of size or the risk of distortion, it may be sufficient in some cases to temper for 4 hours at 850°C followed by air cooling. This reduces the risk of intergranular corrosion but is by no means as efficient as full solution treatment (ref. 13). At this juncture it might be of interest to point out that the low carbon and stabilized grades of steel are now used on a very wide scale, and post-heat treatment is seldom carried out. It should be noted however, that, the risk of intergranular corrosion is not confined to the heat affected zones near the welds. A single cycle of fusion followed by solidification is usually insufficient to sensitize the weld metal to the same extent as other cycles in which the maximum temperature is below the melting point. Nevertheless, local sensitization will almost inevitably occur if a weld is reheated, as when other deposits are made adjacent to it. These conditions are encountered for instance at weld intersections; again, when a circumferential joint is made and the welder arrives back at the starting point, which by then will have cooled down, local sensitization will occur. In other words, the filler metal must itself be of a stabilized grade. The choice lies between very low carbon and Nb stabilized steels-titanium is useless because it oxidizes and the weld deposit will not contain sufficient for stabilization. The best solution, however, is to select a filler metal with a balanced composition that results in a duplex austenite-ferrite weld meal structure. Duplex structures must on the other hand be avoided if intergranular corrosion is not the only consideration. Corrosion resistance in certain media, e.g., boiling nitric acid solutions at concentrations in excess of 20% I-IN* is contingent upon a fully austenitic structure in both parent and weld metals. This limits the choice to austenitic filler metals with very low carbon contents or Nb stabilization (in addition to bare wire for argon-shielded arc welding, coated electrodes are now available that deposit weld metal with less than 0.02% C). Great care must also be taken to avoid weld cracking which would be initiated at high temperatures, after the metal has solidified. Generally, crack sensitivity is a function of the weld metal composition. Experience has shown that welds with a duplex austenite-ferrite structure are immune to cracking. Ferrite is more ductile at very high temperatures and the metal consequently accommodates itself more easily to the stresses that are usually at the origin of the cracking. The most popular types of duplex electrode contain 0.03-0.06% C, 18-21% Cr. 8-10% Ni and 2-3% MO, the steel is known as 20-10-3. Attempts are sometimes made to control the ferrite content of the weld metal within a certain range; the minimum corresponds to good crack resistance at a specified severity of constraint and the maximum to specified mechanical properties or corrosion resistance. The range commonly adopted is 3 to 6%.
415
So far we have been discussing cracking in the weld metal itself. Occasionally, though far less frequently, cracking may be observed in the parent metal immediately alongside the fusion boundary. Cracking may still occur if the well metal is a crackresistant duplex type. The trouble is quite specific to steels containing niobium and does not occur in the Ti-stabilized grades. It is also known that cracks can propagate through the heat-affected zones if the weldment is reheated; in other words, these cracks can occur during stress-relieving heat treatments (550-950°C) or after some service at elevated temperatures. For further information, the reader is referred to a number of references to the welding of stainless steels (refs. 5-16).
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