CORROSION, the BILLIONDOLLAR THIEF ZII.
The Testing and Prevention of Corrosion
FREDERICK A. ROHRMAN Michigan College of Mining and Technology, Houghton, Michigan
This, the concluding article of the series, has for its and complex, organic inhibitors of modern technology. The desirability of predetermining the adequacy of purpose the presentation of the methods for teszing and preventing corrosion. Most readers are familiar wiMt metals and alloys for definite uses has led to the imporsome methods for preventing corrosion although they may tance of mrious corrosion tests. The tests, as described, not be acquainted with all of the methods employed. may be actual plant tests, field tests, or accelerated tests. All the practical methods for preventing or reducing cor- A number of the latter are discussed i n detail. Reasons rosion are outlined and described, from the tars and are given for careful test interpretation and conservatism pitches employed by the Romans to the metal spray guns i n drawing conclusions.
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Nature i s the noblest engineer, yet uses a grinding economy, working u p all that i s wasted today into tomorrow's creation. Emerson THE PURPOSE OF CORROSION TESTS
N E of the great branches of chemical engineering is the study and application of metallic or nonmetallic materials of construction. The diversity of artificial and natural sources of corrosion has made an important field for the man trained in chemistry, metallurgy, and engineering, and probably also in economics. What sort of steel should be employed for bridge construction in the humid regions of Ceylon? Wbat metals will prove themost satisfactory for pumps - and pipe lines in k z o n a copper mines or Bulgarian oil fields! The public has suddenly demanded a new food product recluires soecial chemical treatment:what ma. terials wii be us& for processing this food so as to avoid contaminating i t with corrosion products? All these problems, and many more, must be solved in one way or another. Metals or non-metals may be employed in a "bit or miss" fashion until a satisfactory material is found, or they may be tested under condkions as nearly like the actual as possible, or the general resistance of the material to certain other corroding media may suggest its applicability. The first method carries all the odium of a confession of ignorance, yet i t is that employed on most of our materials of construction, up to this time and today also. In order to eliminate the waste, confusion, and cost of the method just mentioned, so-called "field tests" were developed. By this method one could use samples of different materials under service conditions for a period of days, weeks, or mopths to observe their behavior. This is a very logical and simple method, but because of the time element it too is not altogether satisfactory.
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Having a material and knowing its general properties, why can one not determine its suitability for a certain purpose by means of accelerated tests in very corrosive media? In other words, if a material is found to be resistant to many acids and salt solutions, why can it not be used for a purpose where the corrosion is not likely to be.so severe? This reasoning has led to a number of tests called "accelerated tests." These latter methods immediately found many followers and stimulated proponents of other accelerated tests. ACCELERATED CORROSION TESTS
The best known accelerated corrosion tests are: 1. 2. 3. 4. 5. 6. -
Electromotive tests Total immersion tests Partial immersion tests Wet and dry tests Salt and acid spray tests Color tests -
Electromotive tests, such as those devised by Todt,ss attempt to measure the potential and current between the sample and solntion or between the sample and a noble metal, or to make the specimen act as anode under a specified potential for a definite time, after which the loss in weight is determined. The potential relationship will give, of course, the tendency for the sample to go into the solution used and under such conditions as desired. According to Faraday's laws, the current will be the criterion of the amount of metal going into solution (taking into consideration any anodic gas evolution).
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6 8 T o ~ r KorrnsiDlt , Metallsckuta, 3, 37 (1928); 2. Electrochem.. 34. 586 (1928).
Hadfield and Newberys8obtained satisfactory results detected by qualitative means. Chief among these is by means of single potential tests upon steels. Feuer,eQ the ferricyanide or ferroxyl test6' The most modern use of this test is upon iron- or steelin a similar manner, using brasses and bronzes, obtained results which were comparable with those obtained by plated work; paper is dipped in a solution of NaCl and ferricyanide and applied to, say, the tin-plated iron. other means. The various immersion tests, though probably the Where any porosity is present or the tin has corroded simplest to perform, are yet the most difficult to inter- to the iron a blue spot will appear, indicating that the pret correctly. During a total immersion test the test femcyanide is reacting with the products of the iron pieces are completelv immersed in the corrodinrc medium corrosion. selected. In the partial immersion test, wgich is of THE VALUE OF ACCELERATED TESTS wider application, the effect of oxygen a t the mediumair line can be noted. This factor is of special imporVernone2in a simple manner summarized corrosion tance in cases where equipment or material is partially testing when he stated, "Corrodibility is not an intrinsubmerged in corrosion media. The last of the immer- sic property of a given metal or alloy; i t is to be resion tests, the alternate wet and dry test, or the inter- garded equally as a function of the particular corroding mittent test as i t is sometimes called, subjects the medium; i t is also affected by various extraneous samples alternately to a total immersion and to a dry- factors." Evans" is just as logical when he statesing period. "There is nothing essentially unsound in seeking for an Before most of the immersion tests, the samples are accelerated test. But the acceleration must be obweighed, so that after the test a re-weighing gives a loss tained by intensifying the adverse factors which will or gain of the sample. Often it is necessary to remove be present under service conditions, not by introducing the corrosion products which have formed upon the new factors." That these statements are correct has test pieces so as to determine the extent of the como- been amply proved by any one familiar with the subsion. The loss in weight is generally translated into ject. It does not sufficeto subject a metal to a medium terms of loss per unit area per unit time. Sometimes to which i t is to become exposed; the many factors the density of the material is also taken into considera- affecting corrosion, as described in Part Two,* must be tion, and the depth of penetration per unit time is de- considered as well. At the present time the author is studying the cor termined. Conclusions from these last determinations are generally fallacious because corrosion rarely pro- rosion of various metals and alloys in hydrochloric ceeds uniformly over the surface. Pits and holes are acid solutions of varying concentration. In all these formed by local action, and a specimen which is as- tests oxygen plays a most important rdle, so that all signed a corrosion life by this method will almost invari- attempts are made to obtain the most severe oxygen concentration. Once this is done the maximum intensiably fail much sooner. fied corrosion rate is obtained and the behavior of the probably as important as the quantitative conclusions are the qualitative conclusions. The appearance metal toward hydrochloric acid solutions can be conof stains, pits, and hydrogen evolution, the color of s-ativel~ determined. If the metal were merely the corroding medium, and the formation of corrosion placed in the HCI solution for a definite period and a products, all are criteria of the corrosion reaction. loss in weight determined, a conclusion might be obSometimes after the test a physical strain will be im. tained which would not necessarily follow under adverse posed upon the metal so that any change in physical conditions of oxygen concentration. Most alloys conproperties can be noted. This test is especially impor- taining nickel are extremely sensitive to oxygen contant in cases where strain is likely to be a factor in the centration. One can easily double and triple the corfuture use of the material. The extensive researches rosion rates of these alloys by choosing different oxygen of McAdam of the U. S. Naval Experimental Station concentrations. So, often, iron or steel to be used for water pipes or outdoor uses is given an accelerated have been very enlightening on this subject. test in an acid or salt medium. The results of these In order to simulate tropical conditions a test called the "alternate hot and dry, cold and wet"test has tests and of actual use are generally contradictory beheen devised whereby the humidity and the tempera-cause of the difference in the conditions presented. In ture are varied between wide limits at regularcycles, these tests any oxide film or rust will be affected by the accelerated tests are solutions, while in the air or water the oxide film may probably the most satisfactory the salt and acid spray tests. The samples are snb- be protective to the A" important error in reporting corrosion tests has jected to a fine spray of salt or acid by means of an atombeen stressed by Palmaer.64 The corrosion of a metal izer. sarav testersvarv considerablv in desien. but " . they alih&e for their h o s e the formation of a fine ''"VERNON. WALEER,J. Ind. Eng. Chmn.,1,295 (1909). mist which will come into contact with the test pieces. "A bibliography of metallic corrosion." Arnold & The color tests depend upon the production, by the Co.,London, 1928, p. 59. EVANS, "The Grst report of the corrosion committee." Iron corrosion reaction, of chemical changes which can be and Steel Research Council, 1931, p. 56. PALmEn, "The corrosion of metals," Svenska Bokhandels'* HADPIELD AND NEWBERY, PYOC. Royal Soc.. 93A, 56 (1917). centralen, 1931, vol. 11, p. 129. FEVER, Chem. Met.,22, 1197 (1920). See p. 215ff.,April J. CHEM.EDUC.
generally proceeds slowly a t first and then speeds up to a maximum (the period of induction). If the test is of short duration the maximum corrosion rate may not be reached. In short, corrosion testing is easy to perform but difficult to interpret. All such tests should be reported under every factor possible and any adverse factors should be intensified so as to give "conservative" estimates as to the corrodibility of the material in question. Tests should run long enough to pass the period of induction. The qualitative aspects of the test should be considered as well as the quantitative aspects. CORROSION PREVENTION
"Save the surface and you save all" has correctly become a popular byword. Since corrosion reactions take place i t t h e surfaces of metals and alloys the protection of the surface is the prime objective in corrosion protection. Corrosion may be prevented or retarded by the use of thefollowina methods:
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Alloying Removal of strains Polishing Producing insoluble oxide coatings Producing insoluble phosphate coatings Electroplating Covering with other than by plating Covering with non-metallic materials Making cathodic with force Treatment of corroding medium Addition of inhibitors to the medium.
which are among the only alloys capable of resisting hydrochloric acid; nichrome, containing nickel and chromium, a heat-resistant as well as corrosion-resistant alloy; and illinm, a very complex alloy, used for calorimeter bombs. There are many others too numerous to mention. Just why a particular alloy possesses such useful properties with respect to corrosion-resistance cannot be answered in this paper. With one group of alloys the resistance may be due to homogeneity, and with another to the ability to form impervious oxide films. On this phase of the topic many differing theories have been advanced. 2. Removal of Strains.-As mentioned in Part Two, strains offerfocal points for corrosion reactions. These strains can be removed by heat treatment and annealing. Many manufacturers are resorting to this procedure to improve the corrosion-resistance of their fabricated materials. 3. Polishing.-Any form of polishing which does not produce surface &rains should make a metal more resistant to corrosion. A smooth surface will possess fewer crevices and indentations, which form differential oxygen concentration cells. Moreover, a smooth surface will actually have a smaller surface for the corroding medium to contact with than a rough surface. Industry has taken advantage of this value of polishing, until today most of the metal work that one comes in contact with possesses a bright finish. 4. Producing Insoluble Oxide Coatings.-It is perhaps fortunate that nature has endowed the more anodic metals with oxides which resist solution. Aluminum, chromium, nickel, and others form oxide films which tend to protect the underlyin~metal. The ferrous metals have the -same inclination, but are perhaps more temperamental in this respect. Corrosion-resistant oxide films can be produced on iron and its alloys by natural or artificial means. Some of the common methods are to heat the metal a t a definite temperature and under such conditions as to acquire an adherent film of Fezor. The Badf, Bower, and Gesners5 processes take advantage of this principle. The Ruffington process consists in placing the metal in a molten mixture of potassium or sodium nitrate and manganese dioxide and then exposing it to the vapors. The same oxide is formed as in the other processes but the temperature is not so high.=' Metals may be treated electrolytically so as to form insoluble oxide films. Bengough and Stuart67 made aluminum and aluminum alloys anodic in a chromic acid bath. Fink and KenneysS used the same method in making "stainless" steels more resistant to corrosion. 5. Producing Insoluble Phosfihate Coatings.-Ferrous metals can be so treated with phosphoric acid that insoluble iron phosphate is formed. This compound
It can be seen that this group arranges itself into factors pertaining (1) directly to the metal, (2) to the medium, and (3) to external influences. An account of each one of these methods could in itself make a thesis; in this paper a brief description must suffice. 1. Alloying.-Not so long ago, the only common alloys were the brasses, bronzes, German silvers, and pewters. Today an enumeration of the iron alloys alone would run up into the hundreds. Some of the most important of the corrosion-resisting alloys are the so-called "stainless" steels containing 1620% chromium, 8-12% nickel, 0.05-0.15% carbon, and a maximum of 0.05% silicon and manganese. They go under diverse names and trade-marks because the patents are licensed to more than a score of companies all of which market the material under their own names. The uses for these alloys are very wide; automobiles, kitchen equipment, food processing, and hundreds of chemical plants are dependent upon them. The alloys of iron containing 12-18y0 silicon (Duriron, Thermisilid) possess excellent corrosion resistance for chemical manufacture. Monel metal, an alloy of approximately " SANG, ELecirochem. Met. Ind., 7 , 351 (1909). 67% nickel and 28% copper, has found manifold use POLL^, "The causes and prevention of corrosion," Ernest in food-processing equipment and chemical manufac- Benn. Ltd.. London. 1924. D. 189. BENGOUGH AND'STUART. see ref. 61, p. 303. turing. Some of the more unusual alloys are the Has"FINK AND KENNEY,Trans. A n . Electrochen. Soc., 60, 235 telloys, containing iron, nickel, and molybdenum, (1931).
adheres to the iron surface and tends to protect the iron from further attack. This is the basis for the Coslett and Parker processes, which were mentioned in Part Two. The latter process, now known as "Parkerization," is used on the body-work of many well-known automobiles. Not only does the phosphate coating protect the underlying metal but it also makes a good base for paints and lacquers. 6. Ekctrop1ating.-Almost any metal can be electroplated with any other metal. A metal may be plated with one that is more electropositive or with one that is more electronegative. In the former case, iron may be plated with tin, silver, and gold, and in the latter case with zinc, nickel, and aluminum. When a metal is thoroughly covered with a more electropositive metal i t is protected from corrosion by virtue of the noble character of the electropositive metal. Examples of this are common silver-plated and gold-plated objects. Tungsten, tantalum, and the platinum metals are also used for plating less noble metals. The great disadvantage of noble metal coverings becomes evident when the deposit is tom or scratched off or when it fails to cover the surface properly. Any such condition causes local-action currents to be set up, and the underlying metal corrodes much faster than if it had not been plated a t all. The elimination of "pin holes" due to improper deposition is a problem constantly facing the electroplater. Tin plate is very satisfactory as a protection for iron and other anodic metals. Tin itself is not a noble metal, but i t forms an oxide film which is very resistant to corrosion. Moreover tin is so close to iron in the electromotive series that if a break or tear appears, local action will not proceed very rapidly (Fe = -0.46, Sn = -0.14. Hydrogen overvoltage on Sn = 0.44). Cadmium-plated iron is often claimed to be very effective because of the low potential between cadmium and iron (Cd = -0.40). Plating with a less noble metal results in corrosion protection by another means. Zinc-plated iron is useful because when a break between the two metals occurs, only the zinc will corrode, so that a t the expense of the zinc the iron is protected. Chromium-plated ware acts similarly. The art of electroplating is complicated by the fact that many problems are encountered in making the deposits "pin-hole" proof, adherent, resistant to shock, etc. A chromium-plated object does not consist merely of a chromium plate; usually i t is a chromium plate upon a nickel plate which in turn is deposited upon a copper plate. The copper plate acts as a soft metal cushion, while the nickel plate gives a deposit to which the chromium is more adherent. A thorough discussion of electroplating is given by Blum and Hogabloom.B8 7. Metallic Coverings Other Than E1ectrodeposited.Protective metals may be applied to other metals by means other than electrochemical, as by: dipping in BLUMAND HOGABLOOM, "Principles of electroplating and elebrofo~ming," McGraw-Hill B w k Ca., New York City.
the molten metal, spraying with the molten metal, and heating in the presence of metal or metal oxide powders. When a clean metal is dipped in a molten bath of another metal i t acquires a coat of metal. If the molten metal is zinc the process is known as galvanizing and the coated metal is known as galvanized ware. Tin may also be applied to metals in a like manner. S ~ h o o p was ' ~ the first to discover the metal-spraying process whereby a molten metal is sprayed upon an object so as to cover i t uniformly with a thin coating. The convenience and ease with which this process lends itself to practical work makes it very valuable in industry. Zinc is generally the only metal so applied. When a metal is heated in the presence of another metal there is a tendency toward surface alloy formation. This is known to the metallurgist as cementation. (It should not be confused with electrochemical cementation. See Part One.) When the lower melting metal is in the powdered state this tendency for surface alloy formation is increased. .The use of powdered zinc to alloy with metals and form a zinc surface is known as "Sberardizing," after Sherard CowperC ~ w l e s ,the ~ ' first to apply this method. The same method is used to cover metals with aluminum, and results in better heat resistance. This process, called "Calorizing," is used for covering iron, steel, nickel, copper, and brass. When a chromium cover is obtained it is called "Chromizing." Silicon, boron, tin, tungsten, molybdenum, tantalum, and many other elements have been applied to metals by this process. 8. Covering with Non-metallic Materials.-The most ancient and some of the newest means of protecting metals involve the use of various organic and inorganic covering materials. The early Romans employed paints and asphalt for protecting iron work. Some of the more recent applications are the use of rubberlined tanks and equipment for handling hydrochloric acid, enameled steel work for the preparation of certain pharmaceutical products, and various new lacquers and paints. In this group would also be included glass, oils, tars, cements, concretes, etc. Some materials, such as silica and bakelite, have such satisfactory physical properties that in many chemical operations they are used directly. The subject of paints is an interesting one because of the difference in behavior of the various kinds that are on the market. Paints vary in electrical conductivity, impermeability, adherability, elasticity, etc. It is poor practice to use a bright red oxide or barium sulfate paint for a primer, because these compounds are conductors of electricity and would promote corrosion a t any breaks or holes. Generally a priming coat consists of a zinc oxide or of white lead compounds, which are poor electrical conductors. Over the first coat of paint i t is desirable to apply a paint characterized by its impermeability and water-shedding ability rather than its non-conductivity.
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"SCHOOP, Ra. met.. 7,585 (1910). COWPER-COWLES. ElecLrochemisl & Melallurgist. 3, 828
(1904).
9. Making Cathodic with External Eledromotine Forces.-The electromotive forces in a corrosion reaction are very small, generally in the order of tenths of a volt. Through the application of an external electromotive force of a greater magnitude and in the opposite direction, anodic elements will naturally become cathodic, and undesired corrosion can be eliminated. Mention has already been made of nickel pasteurizing tubes which were protected by inserting the less noble aluminum metal strips in the milk. The actual use of E.M.F. applied, in situ, during service is sometimes resorted to, the "Cumberland Proces~"'~being a good example. A potential of 6 to 10 volts and of low amperage is forced from iron anodes to the metal to be protected. The success of this idea depends upon the even distribution of the current to that metal and upon the slow rate of anode disintegration. 10. Treatment of Corroding Medium.-The corroding medium may be so treated as to change its hydrogen-ion concentration, its conductivity, and its oxygen concentration, factors that are of fundamental importance in determining the influence which the medium shall have upon anodic metals. The hydrogenion concentration of water, though in the order of only to-' gram ions per liter, is yet sufficient to enable the hydrogen to displace many metals. By the addition of small amounts of soluble carbonates or hydroxides the hydrogen-ion concentration can be forced to such a low figure that the resulting hydrogen potential will be more basic than most metals; consequently the normal corrosion processes will not be able to manifest themselves. Water almost always contains dissolved gases, of which oxygen has the greatest influence upon corrosion. The importance of oxygen as a depolarizer and as a metal oxidizer has been previously mentioned. Since the presence of oxygen is so necessary for most corrosion reactions, means have been devised whereby it can be
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" CUMBERLAND, Trans. Faraday Soc., 11, 277 (1916).
removed. These means are called de-activation and de-awation. De-activation is most often accomplished by allowing the water to come in contact with iron or steel scrap so that the oxygen will be consumed during the corrosion of this material rather than the pipes or boilers later on. Many plants employ this system for removing oxygen from water. De-aeration is the physical method of removing gases from water. Since most gases are less soluble in hot water than in cold, they can be removed by preheating the water in special tanks or holders called de-awators. The solubility of oxygen per 100 g. of H20is 0.007 g. a t O°C. and 0.001 g. a t 100°C. Although not all the oxygen is removed, its influence is nevertheless considerably decreased. There are a very great number of industrial de-aeators, some of which treat millions of pounds of water per hour. Combinations of deactivators and de-aerators are in nse also. 11. Addition of Inhibitors to the Medium.-The use of inhibitors, in a practical way, is of recent origin. The addition of certain organic materials to pickling baths so as to prevent overpickling is comparatively well kn0wn.7~ There seems to be a very great opportunity for research in this branch of corrosion prevention. Lauer" has recently completed a vast amount of experimental work on the use of various organic inhibitors in retarding corrosion. ACKNOWLEDGMENTS
The author wishes to acknowledge the very important criticisms and aid in editing given him by Professor Lisle Rose of this institution and by Dr. Nelson W. Taylor of the University of Minnesota.
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SPELLER. "Corrosion. causes and orevention." McGraw-Hill Book Co., N ~ WYork ~ i t i 1926, , p. 279; SPELLER AND CHAPPEL, Trans. Am. Ind. Chem. Eng., 19, 153 (1927); CREUT~ELDT, 2. anorg. Chem., 154, 213 (1926). LAWER, Thesis, University of Minnesota.
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(This article concludes the series begun i n the March issue.)
