WATER
CORROSION CONTROL I N WATER SYSTEMS' Herbert H. Uhlig
SODIUM SILICATE AS A CORROSION INHIBITOR Leo Lehrman and Henry L. Shuldener
1765
COOLING WATER PROBLEMS I N METROPOLITAN NEW YORK2 Sidney Sussman 1740
PH0S PHAT E-C HR 0 MATE T R EAT MENT FO R METALS H. Lewis Kahler and Philip J. Gaughan
1770
EFFECTS OF VELOCITY ON CORROSION
PROTECTIVE FILM FORMATION WITH PHOSPHATE GLASSES G.B. Hatch 1775
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1736
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H-
Copson
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EFFECT OF TEMPERATURE ON CORROSION Norman Hackerman.
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1745 1752
CORROSION CONTROL W I T H ORGANIC INHIBITORS J. N. Breston 1755
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CORROSION INHIBITORS FOR STEAM AND CONDENSATE LINES R. C. Ulrner and J. W. Wood
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I 2
1761
Presented at Illinois Water Resources Conference, 1951 Pleented a t 12ew T o r k ~leeting-in-r\.liniature,1952.
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INHIBITION OF GALVANIC ATTACK PHOSPHATE GLASSES G. 6. Hatch
WITH
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CATHODIC PROTECTION OF STEEL Leon P. Sudrabin and Henry C. Marks.
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PITTING CO.RROSION CHARACTERISTICS OF ALUM1NUM3 P. M. Aziz and Hugh P. Godard.
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J
1780 1786
1791
Presented at Sational Hewavh C n , i n r , i l of Canada. Oltan.a, 1952.
Corrosion Control in Water Systems HERBERT H. UHLIG Mosrochurettr lnstitufe of Technology, Cambridge, Mors.
OSSES through corrosion of metals pro!mbly first received attention a t the turn of this century. d l e n utilization of iron and steel took a sharp turn upxard. The estirnatrd figures for these losses had a certain interest as statistics, b u t it often remained easier, practically, to replace corroded structures than to trace the source of deterioration. Today the emphasis has changed, and the need for corrosion control is accented particularly as metal supplies hecome short, as ore reserves look emaciated, and as modern industrial equipment must he better protected against increased operating pressures, temperature?, arid other corrosion factors. Certainly the economicss of the situation leaves no doubt that we should know more ahout corrosion than we do, Replacement coats for corroded equipment have become inore rather than less disturbing, and shutdoxns, contamination of products, loss of efficiency, and accidents have never been popular. Hiding ignorance by aniple overdesign to take care of the unknown in
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1736
corrodion is less acceptable eiigiiieering pra \vas in times past. pi ye line.^, pumps, and iiiust no^ entail the optimum erono:nic design meeting both niechanical specifications and expected corrosiori over a long range basis. The acceptable approach to corrosiori problems, in other word$, ha,s become more subtle than merely increasing t,he cross-sectional area of metal. Cathodir prot'ection, metal coatings, organic coatings, inhibitors, treatment of the environment, and alloys offer ved advantages in the saving of metals, , Much has been done in recent years human effort, and dol to mitigate corrosion, but ~tcrossthe board in all industries and wherever metals are used, the major advances along them lines are still to come. What we presently knoir about corrosion mitigation has accumulated through several sources, including (1) basic research, and ( 2 ) service data combined Lvith empirical probing. Stainless steels for example, were dircovered in England by the empirical
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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-Cormsion
by Water
available or obtained later, confirmed the laboratory deductions, probing of H. Brearley in 1912. The use of modern protertive It was this research that also explained why heat treatment, paints and chromate inhibitors also resulted from probing, corninternal stresses, and surface preparation of a steel, despite bined with service data. On the other hand, modern boiler water erroneous intuitive roncepts, have practically no effect on the treatment, the application of the saturation index to corrosion corrosion rate in natural waters. control, the discovery of cathodic protection, and the improved I n the acid region (below pH 4 to 6 depending on the nature corrosion resistance of present-day aluminum alloys were thc of the negative ions), on the other hand, where hydrogen evoluresult of planned experiments and fundamental knowlcdge of tion is the major cathodic reaction, i t was demonstrated that the sciences. composition of iron and steel becomes important. The conFundamental research in corrosion began in the early 1800’s trolling reaction at the minute cathodes of the steel surface in with the basic work in electrochemistry of Sir Humphrey Dsvy, this region of p H is determined not so much by oxygen concenMichael Faraday, and W. H. Wollaston in England; A. Volta tration as by hydrogen over-voltagc. For this reason, surface in Italy; and A. de La Rive in France (8). The quantitative finish, inclusions, alloying elements, and presence or absence of relation between electricity and chemical change in a cell was stress play a part. Therefore, although any iron or steel m a y firmly established; from this the suggestion followed t h a t corbe equally suited t o fresh or salt xaters, some attention to comrosion of metals must be similar to the action that takes place in position is worth while when specifications are set up for R the galvanic cell. steam-return line handling hot carbonic acid or for similar inCathodic protection, one of the most effective, practical stallations. present-day means for reducing corrosion of metals, had its beginning at this time in the systematic research of Davy. His Cast iron corrodes by so-called graphitic corrosion whereby tht. iron constituent of the alloy is converted t o corrosion products. laboratory experiments showed that zinc, tin, or iron diminished these acting as a cement for residual graphite flakes. A corroded corrosion of copper to which it was electrically coupled in a cast-iron pipe, therefore, may have lost most of its mechaniral salt solution, a n idea which he applied later t o protect the strength but is still serviceable for the purpose first intended. In copper sheathing of a British warship. this respect, i t may last longer in some applications than steel 01 iron It is also reported that a 3% chromium steel is subjert to Although this early work established the foundations, it was slightly lower weight losses and less pitting than other low alloj not until about 75 years later, a t the turn of the 20th century, steels in natural waters. that the subject received further attention. By this time, the inThese are only a few instances showing the relation bedustrial age was well under way with its tremendous demands on tween basic research in corrosion, which began 150 yeais metals, and corrosion began t o make a n impression as a subject of ago abroad and 50 years ago in this country, t o moderii technical and economic importance. Whitney (18),in 1903, while industrial development and to a high standard of living. The\ a member of the teaching staff of Massachusetts Institute of introduce the next section of this paper because throughout the deTechnology, focused attention on the electrochemical procvelopment of methods for combatting corrosion, fundamental reesses attending the corrosion of iron; these were followed a search has played an indispensable role. And more important, n c few years later by the researches of his colleague Walker (14, 15). must look almost entirely to fundamental research for major adWalker and his collaborators proved the important role of disvances in the future. I n all fields of technical development, when solved oxygen in the corrosion of iron, and showed t h a t carbonic the obvious trial and error methods reach the point of diminishacid was not necessary to the reaction, contrary t o the general ing returns, only systematic and unprejudiced scientific search impression up to t h a t time. This fact constituted one of the For further truth offers real hope of progress. Advances in corroimportant steps in modern boiler water treatment, making possision control are no exception. ble the economic production of high pressure steam in boilers constructed of steel, from which followed, in turn, cheap electriSurvey of Corrosion Mitigation cal power. Corrosion control was an essential step in this development. A survey of present means for corrosion mitigation diuclows Basic research gained impetus in the 1920’s and included the obvious gaps in the present state of the a r t or science, as the case investigations of Whitman, and those associated with him, on the may be. Until these gaps are reasonably filled, the protection behavior of iron and steel in waters as a function of p H (16, I ? ) . of metals from corrosion will remain incomulete. and the annual This work provided a fundawaste 12) from this mental answer to the agesource will continue to tw old controversy of whether large. The current large ancient iron was better than scale application of cathCorrosion of metals used for waterworks and boiler modern or whether some odic protection has offered operations will continue to present problems for some time, brand of steel or special kind the most recent evidence of iron was better than its that some of the savings and but the over-all losses from this source will decrease as competitor, a question that conservation so much needed basic research in corrosion gains support and as workers service data had seemingly are being accomplished. are attracted to the challenges of a field in which any left open. Within the range Cathodic Protection. Cathdegree of success may amount to millions of dollars saved. of p H of most natural waters odic protection is now applied Cathodic protection should find many economic applicaoxygen depolarization conlargely for protecting buried trolled the rate of corrosion, pipelines transporting oil, gas, tions in controlling corrosion of waterworks equipment and and oxygen c o n c e n t r a t i o n and water and, to a lessw eswill probably be applied more generally during the next was therefore the important tent, for protecting strurturcs 5 to 10 years. Improvement of corrosion protective paints factor. The gross composiexposed t o natural waters, and organic coatings should also b e accomplished in the tion, incidental alloying elesuch as canal gates and indusments, or inclusions consenear future. Of the metals available having greater trial water tanks. By passing quently had little effect, if a current through the water or resistance to attack than those now used, titanium offers any, on the rate. When this soil, of the order of 0.0001 to the most promise. When the price approaches that of the plausible conclusion was an0.1 amp./square foot of struci t will be used in wrought and cast form stainless steels, nounced, a vast amount of ture surface, the actual value and as a clad coating over steel. service data in fresh waters varying with the nature of and in sea water, either then the environment, minute
ill,
August 1952
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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galvanic currents on the surface of the metal accounting for corrosion are neutralized, whereupon reaction ceases. The impressed current for cathodic protection is supplied from a rectifier or generator or may be produced by the chemical energy of galvanic anodes such as magnesium. The life of structures cathodically protected is prolonged almost indefinitely a t the modest cost of installation and electricity or of the sacrificial metal used for galvanic anodes. Cathodic protection for this reason eventually will be applied more widely and will undoubtedly become a factor in prolonging the life of metals used in water and sewage treatment. Galvanized, hot water tanks are now being protected by magnesium anodes contained in the tank, thereby adding several years to their life. Industrial water tanks are frequently protected by an impressed rectified alternating current and corrosion resistant anodes. The Panama Canal gates, covered with a hot bituminom coating, are protected through use of an impressed current of 0.001 amp./square foot and replaceable steel anodes (9). The method is also applicable for protecting the outside of pipes, buried or totally immersed, whatever the corrosive medium in contact. Perhaps it is worth mentioning that when so applied it has no effect on corrosion of the inside surface. In order t o protect the inside, it would be necessary to insert a conforming anode reaching the entire length of the pipe, which obviously is not easily done. I n addition, reaction products a t the anode or of the anode itself are retained within the pipe and must therefore exert no undesirable effects on the water or water solutions carried by the pipe system. It is present practice t o apply an organic coating conjointly with cathodic protection, thereby reducing the required current for protection and better distributing it. Since the coating is insulating, the current flows only to breaks or pinholes in the coating, which are also the areas where corrosion occurs. One magnesium anode buried in the ground can often protect 5 miles of a coated buried pipeline but would be effective in protecting perhaps only 10 to 100 feet of uncoated pipe. Because alkalies are one of the reaction products a t the cathode according t o the reaction
2H20 I/*
02
+Hz
+ 20H-
- 2e or - 2e
+ HtO --+20H-
the organic coating must be alkali resistant. Cathodic protection is not effective above the soil or water line since the protective current can no longer reach the metal surface. It can be applied t o metals other than iron and steel, with the one precaution t h a t amphoteric metals like aluminum, zinc, or lead, which are attacked by alkalies, must not be overprotected by use of high currents, since the accumulated alkali will attack the metal. With iron or steel, overprotection is not critical because aIkalies act as corrosion inhibitors. It is not often appreciated that cathodic protection, in addition t o being useful for protecting metals against rust or general surface attack, also prevents dezincification, pitting, intergranular corrosion, corrosion fatigue (that portion of fatigue accelerated by corrosion), and stress corrosion cracking. Stainless steels, for example, which invariably pit on exposure t o sea water, are immune when cathodically protected. Likewise, brass which dezincifies or which may season crack (stress corrosion crack) when stressed in certain environments can be protected by this means. The possible applications of cathodic protection in specific instances of this kind are legion and have only begun to be appreciated. However, the basic science of this approach t o corrosion control trails behind practice. Urgently needed today are quantitative data relating corrosive factors of the environment, such as pH, velocity, dissolved salts, galvanic couples, surface finish, and temperature t o the minimum required current for total protection. Also needed are fundamentally established criteria of
1738
complete proteetion more satisfactory than those now in use. Overprotection, with its added costs or damage in some instances, would then be avoided. It is reasonable to expect improvement in the efficiency (reduction of local corrosion) of sacrificial anodes above the present 50% figure and the development of more efficient insoluble anodes, having lower oxygen overvoltage, in applications where impressed currents are applied. These are but a few of the many problems. Metallic Coatings. The metallic coatings used in largest quantity are tin, zinc (galvanized), and nickel coatings on steel. Tin coatings are employed largely for food and other containers (tin cans) for which they are singularly well suited from the standpoint of corrosion protection and lack of toxicity. Zinc coatings are relatively effective in avoiding rust of iron whether exposed t o the atmosphere, buried in the soil, or immersed in water. The life of the coating increases with coating thickness. The principle of protection is the same as that explained under cathodic protection, the zinc corroding sacrificially and the resulting current protecting iron cathodically. Only in some hot waters above approximately 140" F. docs zinc fail t o protect iron, a fact only recently established by laboratory experiments (IO) and confirmed by service data ( 1 ) and further laboratory tests ( 3 , 6). This is another illustration of how fundamental work in this field points the way, whereas service tests requiring so long a time before evaluation and involving so many uncontrolled variables during the test often fall short of being conclusive. Nickel coatings are noble t o iron in the galvanic series and must be relatively pore-free in order t o protect the base metal from attack. This is accomplished by plating a sufficient thickness of metal usually of the order of 0.0005 to 0.0015 inch. Presumably, much better protection will eventually be achieved for the same thickness of metal when commercial plating practice makes it possible t o electrodeposit nickel with fewer numbers of pores. This is a reasonable objective of presentrday plating research, and is receiving some attention. Clad coatings, such as stainless steel or nickel bonded metallurgically to steel, or pure aluminum bonded to a stronger aluminum alloy, are free of pores and offer protection equivalent t o the bulk metal constituting the overlay. Some design problems are encountered in handling edges of such composite materials where the underlying metal is exposed, but, in general, the problems can be handled satisfactorily. The costs run high compared with electrodeposited coatings. Titanium-clad steel may be the ultimate answer in many applications involving exposure to salt solutions or hot waters, particularly as the price of titanium is lowered. Unlike 18-8 stainless steel, this metal has been resistant to crevice corrosion and to pitting in sea water in all tests conducted so far. Further developments in metal coatings may eventually provide (1) new types of sacrificial coatings in the form of alloys, (2) reduced porosity of noble-metal coatings, (3) cladding processes less expensive than those now available, and (4) sprayed metal coatings with satisfactory impermeability and lower in cost than present coatings of this kind. Sprayed coatings have the inherent advantage that they can be applied to finished structures. Organic Coatings. Compared with their good performance in atmospheric exposures, the common drying-oil paints have been only fair when applied t o steel structures totally immersed in natural waters or buried underground. Their life is short even under ideal conditions. Multiple coatings, baked on, of some synthetic resin paints are better but expensive, and it is seldom possible t o guarantee that all edges or corners are coated adequately, or will remain so during service. Phosphating of surfaces before painting has usually proved beneficial in improving protective qualities of all paints. Heavy bituminous coatings remain the standard for buried pipes and are also being used on the inside of water mains. Heavy plastic
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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Corrosion by Water coatings and rubber, bonded t o steel, offer good protection to severely corrosive chemical environments where they are used with economic advantage. One of the most chemically resistant of modern plastics is the tetrafluoroethylene type, which resists boiling acids and alkalies, all solvents, and corrosive gases like chlorine. It is so successfully resistant t h a t bonding i t t o metals has presented a major problem. It is a reasonable expectation t h a t materials of this kind will eventually enjoy wide application in many industries. Perhaps their superior resistance will eventually be incorporated into paints having protective qualities marking an improvement over present paints by several orders of magnitude. Among the inexpensive coatings, portland cement continues to offer outstanding protection to iron and steel against hot or cold waters and to the soil. Water pipes in service for 60 years or more are not uncommon. Glass-covered or enameled steel surfaces also offer good protection but are easily damaged mechanically or when subject to thermal shock, making them applicable largely to specific chemical environments or to atmospheric exposures Inhibitors. Chromates and nitrites continue to be the most efficient of our present-day inhibitors for controlling corrosion of iron and steel. They suffer by being toxic and, hence, are not applicable where potable waters or food is handled. They also have limited use at elevated temperatures or where salt concentrations are appreciable. Sodium silicate continues in use for treating potable soft waters, effectively reducing pickup of flocculent rust, as well as reducing dezincification of brass. It has not been outstandingly successful in preventing attack of galvanized domestic hot water tanks. Glassy metaphosphates are being used in a few parts per million to combat corrosion of various steel systems and in particular nonrecirculating cold water systems. Recent work in the Corrosion Laboratory a t M.I.T. indicates that dissolved oxygen is beneficial to the efficiency of metaphosphates in a degree not previously suspected, although the effect has been known (8, 6). The mechanism of behavior of inhibitors such as these and a better understanding of their proper application is one of the present projects at M.I.T. supported by the Office of Naval Research. Basic comparative data on all commercially available inhibitors are needed today relating their efficiency t o temperature, pH, dissolved chlorides, sulfates, nitrates, calcium and magnesium, dissolved oxygen, and organic materials. The disturbing effect of galvanic couples also requires adequate evaluation. There seems to be no doubt t h a t a good, nontoxic, inexpensive inhibitor for potable waters, particularly if it were effective in hot water systems, is presently needed and would be welcomed by the water industry. Alteration of Environment. Deaeration of boiler waters probably constitutes the most important example of mitigating corrosion of expensive equipment by altering the environment. Oxygen removal is also finding use in reducing corrosion of pipes and steel equipment handling cold and hot waters, where the required oxygen content need be only 0.1 to 0.3 p.p.m. instead of virtually zero as in high pressure boilers. This approach may eventually prove worth while for corrosion control of municipal water systems, since it also solves simultaneously the householder’s problem with respect to piping and hot water tanks. The normal taste of the water may need to be restored by an aerator at cold water faucets. One problem may possibly arise in this connection, particularly in applying vacuum deaeration, through the enhanced opportunity for sulfate-reducing bacteria t o accelerate corrosion. These bacteria thrive only in waters of low oxygen content and corrode ferrous metals many times more rapidly than normally aerated water. Their control, in some instances, has been accomplished by means such as chlorination. Deaeration of waters will also reduce corrosion of copper, brass, and lead but not stainless steels or aluminum, the latter
August 1952
depending on dissolved oxygen for their passivity or corrosion resistance t o aqueous media. On this same subject, Groesbeck and Waldron ( 4 ) showed t h a t increased diwolved oxygen, although at first stimulating attack, reduces the corrosion rate of iron in distilled water above concentrations of 16 ml. per liter. This result has been confirmed by others. The practical value of these data has not yet been assayed in large scale equipment, but t h e suggestion occurs t h a t oxygenation of soft waters may be one possible way t o reduce corrosivity of such waters in contact with steel. The mechanism proposed for this behavior is one either of building u p a more protective barrier iron-oxide film or chemisorption of a n oxygen film on the iron equivalent to a monolayer or less (IS). Reduction of corrosion by oxygenation is not expected for copper or brass nor for waters containing appreciable dissolved salts, such a8 chlorides and sulfates, or at high temperatures, because the films responsible for passivity either no longer formor are less protective. It was known for many years t h a t some natural fresh waters are more corrosive than others. The difference eventually resolved itself into the presence or absence of a protective layer on the surface of the metal, usually calcium carbonate. It remained for Langelier in 1936 ( 7 ) t o analyze this problem quantitatively from the standpoint of physical chemistry and to propose as a result the Langelier or saturation index. Waters with positive index are supersaturated with respect to calcium carbonate; hence, they can deposit a protective film and are nonaggressive from the standpoint of corrosion. On the other hand, waters with negative index are undersaturated and aggressive. This convenient classification of waters has led t o a scientific basis for corrosion control of water systems and has proved useful in many instances. Only where conditions are such t h a t calcium carbonate cannot precipitate as a continuous protective film, as when colloids are present, does the rule break down. Also, occasionally heavy scaling occurs i n waters of positive index at elevated temperatures. Further work along these lines will undoubtedly take care of the special situations. Metals and Alloys. Galvanized iron remains the standard material for handling cold and hot waters of positive saturation index. I n some hot waters, galvanized iron may pit more than ungalvanized iron, as mentioned previously. For some waters of negative saturation index, copper and red brass for small size pipe offer economic advantages over steel, whether galvanized or not. Monel (70% Ni-30% Cu) and copper are useful for hot water tanks in soft water areas. Aluminum is satisfactory for distilled water but is not satisfactorily corrosive resistant to all types of waters, especially when the waters have been previously in contact with iron or copper. Magnesium, likewise, is sensitive to impurities i n the water and, as yet, cannot compete economically with t h e heavier metals mentioned. Lead is presently expensive and, although durable, may contaminate soft waters with lead salts sufficiently t o render the water unsafe for drinking. This fact should be borne in mind when water softeners are installed in some of the older cities, where lead piping is still in use. Stainless steels are not used widely because of expense and also because they may pit rapidly in some waters containing dissolved chlorides, especially when ferric and cupric ions are present simultaneously. Titanium holds promise of application in the handling of hot and cold waters, if and when the price approaches t h a t of the stainless steels. Summary and Conclusions
Corrosion of metals used for waterworks and boiler operations will continue t o present problems for a long time t o come, but the over-all losses from this source will decrease as basic research in corrosion gains support and as workers are attracted t o the challenges of a field in which any degree of success may amount t o millions of dollars saved. A reliable, inexpensive, nontoxic inhibitor is needed t o help reduce tuberculation and clogging of
INDUSTRIAL AND ENGINEERING CHEMISTRY
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water pipes, as well a8 to proteot hoi \vntri. taiiki. 13’ailing thir, deaeration of waters a t the source offers some hope and proinise, Intentional oxygenation of caold, soft waters, which are the mosl, corrosive type, may also prove useful in special cases. Lime a,nd sodium carbonate additions, in s c i ~ ) r dwith Langelier’s developnieiit, continue to be a ueeful approach to corrosion control. Cat.hodic protection should find many economic applications in controlling corrosion of waterworks equipment, and will probably be applied more generally during the next 5 t o 10 years. Improvement of corrosion protectjive paints and organic coat,ings, both from standpoint of performance and price, should be :twomplished in the near. future, and will help further to reduce tlie expense and burden 0 1 iiietal deterioration. Of metals avail:~ble to resist attack markiiig :in iniprovement o v e ~t,liose n o i v used, titanium offers the most promise. Wheri the pi,ic:e : ~ p proaches that of the stainless strels, it will find use in \vro\ighl, ant1 cast form and as a clad caoatiiig o v ~ steel. r
Gilbeit, P.’J‘., F’ittCilniryh Ititern. Coni. on Surface Iteactioiis, p. 21 (1948).
Groesbeck, E., and \VaIdion, L., Proc. Am. Soc. Testing M n terials, 31, Part 11, 279 (1931). Hatch, G. B., and Rice, O., IND.ENG.CHEM.,37, 752 (194.5). Hoxeng, R., and Pnitt,on, C., Corrosion, 5, 330 (1949); 6 , 308 (1950).
Langelier, M-.F.,.I. Am, Wuter Works Assoc., 28, 1500 (1936). Lynes, IT.,J . b’Zrct,,ochem. SOC.,98, 3 C (1951). Miles, John A , , presented a t Symposium on Corrosion apousored by Corrosion Subcommittee of Deterioiation I’vevention Cornniittee and Office Yam1 Research, IVashingtoti, 1).C. (February 1949). Sohikorr, G., ’/‘m?is. Elrclrochem. Soc., 76, 247 (1939). Uhlig, H. H., Chem. E n y . .Vru’s, 27, 2765 (1949). Uhlig, H. H., C o r w s i o 7 ~6, , 29-33 (1950). Chlig, H. H., ,I!etuza:c et Corrosion, 22, 204 (1947). Walker, )I7. H., ’/’I.UILY. fllentrochem. Soc., 14, 175 (1908). \Valker, V’. H., Crderholrn, A, and Bent, L., J . Am. Chem. S o c . . 29, 1251 (1907); 30, 473 (1908).
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27B
(1924).
\Vhit,maii. \j’.
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