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by Anton de S. Brasunas ASM Metals Engineering Institute
CORROSION A
W O R K B O O K
F E A T U R E
E C
Corrosion Review for 1958 r^s IN previous years, corrosion research efforts in 1958 have been a many-pronged advance into the pressing corrosion problems of the day. Interest and activity in all corrosion branches continue, from underground pipelines to marine applications and high temperature corrosion by exhaust gases and the air atmosphere. The attack of metal components by missile and nuclear fuels is a serious problem, which remains unsolved because of changes in design, increasing operating temperatures, and ever-changing fuel materials. Activities in Europe during 1958 were noteworthy. Of possible interest to all engineers are corrosion papers dealing with automobile radiators and those presented at the Symposium on the Protection of Motor Vehicles from Corrosion held in England, March 11 and 12,1958. A series of eight articles dealt with the deterioration of car bodies, cooling and wiring systems, and other miscellaneous auto parts. In Brussels, a Corrosion and Packaging Exposition displayed sealed plastic envelopes for guaranteed protection in delivery and storage, cocooning of stored articles, and various coating and chemical treatments. Refractory Metals
• Vital corrosion problems occur in missile technology. Gases in the hot combustion chambers of engines and on the exterior surface of the missile react with metallic and nonmetallic
missile components when the vehicle re-enters the relatively dense earth atmosphere and becomes severely heated. High melting metals, graphite, cermets, and ceramics must inevitably be thoroughly investigated for this application. This means that the alloys of such metals as tungsten, rhenium, tantalum, molybdenum, and niobium (in order of high melting points) must be carefully surveyed, and their ablating characteristics determined. (Ablation is a property of materials which combines heat absorption and heat transfer barrier phenomenon during its removal from a surface by vaporizing, depolymerizing, or other removal phenomena.) Coatings will play an increasingly vital role for reducing heat absorption, corrosion, and erosion resistance, lubrication, and maintaining themselves at high temperatures. Metallo-organic compounds, fluorocarbon materials, sprayed ceramics, chelate polymers, and ferrocene-type structures have high potential. Research at Los Alamos on the potentially important binary alloys of the rhenium-molybdenum system has just been reported (Trans. Am. Soc. Metals, in press; Preprint 72) and is an addition to Hansen's "Constitution of Binary Alloys" (McGraw-Hill, 1958), which has just come off the press. It almost seems a diabolical trick of nature, that metals with high melting points and best suited for use above 3000° F. should possess
such poor corrosion resistance in air and oxidizing environments at these temperatures. Alloying and surface coatings seem the logical approach to this dilemma, and research along these lines, especially with molybdenum and niobium, is progressing with encouraging results. High temperature corrosion by ordinary air continues as an important factor in materials selection. Not only is the corrosion kinetics for the reaction, metal —> oxide, vitally important in missile technology, but the introduction of small amounts of certain foreign atoms into the metal lattice by diffusion from the metal environment may be highly deleterious because of possible embrittling effects. Of the. refractory metals listed above, niobium (columbium) is receiving attention because of its excellent potential usefulness at elevated temperatures for nuclear reaction construction as well as aircraft engine and missile components. Photo below shows how pure niobium is corroded in 4 hours by air at 1800° F. in comparison to nine niobium alloys. Surprisingly, pure niobium and some of its alloys are corroded more rapidly by air at about 1475° F. (800° C.) than at 1830° F. (1000° C.) (Trans. Am. Soc. Metals, in press; Preprint 70). This anomaly has been attributed to a sintering of the porous oxide, Cb 2 0 6 , forming a more coherent and protective layer at the higher tempera-
COURTESY NASA
Appearance of pure niobium and its alloys composed of 2 atomic %
of elements shown, after 4 hours in air at about
1800° F. I/EC
WORKBOOK
FEATURES
93A
duced by changes in chemical composition, or subsurface void formation because of unusual diffusion effects. The phenomenon of liquid metal corrosion has emphasized the need for a better understanding of equilibrium diagrams such as have been compiled by Hansen. Certain observed interactions indicate that the reported absence of compounds or significant solubility in certain binary systems should be reinvestigated. Some data relating to portions of the binary systems of lead with nickel have been published [Trans. Am. Inst. Mining Petrol. Engrs. 212 (1958)]. COURTESY
NASA
Unprotected tungsten, highest melting m e t a l , corrodes r a p i d l y in air even at 2 0 0 0 ° F. The white cloud of " s m o k e " is the volatile o x i d e , W O 3 O i l A s h Corrosion
The accelerated corrosion of metal components above 1350° F. when trace amounts of vanadium-bearing contaminants are present on the metal surface, continues to plague corrosion engineers. Such an environment results from the combustion of certain residual fuels and has been shown to increase corrosion rates more than a thousand times! This problem was reviewed by K. Sachs [Metallurgia 57, 167, 224 (1958) ]. Several actual case histories of failures have been reviewed by members of NACE Task Group T-5B-3 [Corrosion 14, No. 8, 369-72 (1958)]. The practical techniques being used to reduce accelerated oxidation involve protective coatings to exposed metal surfaces, oil additives to raise the melting point of the resulting ash, and design changes to reduce metal temperatures below 1350° F. Other possibilities of reducing corrosive attack are oil treatment to reduce vanadium content and selection of a suitable alloy for improved corrosion resistance. The first is too expensive and the second is unavailable at present. Reactor Corrosion
Aqueous. The interest in various types of nuclear reactors has focused attention on numerous aqueous and nonaqueous corrosion problems. It is recognized that unsolved corrosion problems are a major deterrent to the design of more efficient 94A
reactors, whether gas-cooled, watercooled, or liquid metal-cooled. One area in corrosion technology that appears to be gaining in importance is concerned with corrosion by high-purity high-temperature water. Several uranium-molybdenum alloys have been investigated [Trans. Am. Inst. Mining Petrol. Engrs. 212, 26 (1958)] and decomposition of water resulted in hydrogen absorption by the uranium alloy and precipitation of U H 3 throughout. Other investigators have demonstrated that uranium alloyed with 10% zirconium (by weight) or less is corroded by 600 ° F. water at essentially the same rate as unalloyed uranium; with 15 and 20% zirconium, however, attack by superheated water was reduced by a factor of about 20—i.e., from about 3500 mg./(sq. cm.) (hr.) to about 180 [Corrosion 14, 313 (July 1958)]. Liquid Metal. Because of the important use of low melting metals in nuclear reactors as heat exchanger fluid or as solvent for liquid fuel, liquid-metal corrosion studies continue to be made. The interaction between liquid metals and solid metals, though rarely involving an oxidation reaction, is an accepted field in the science of corrosion. The solubility of a solid alloy or certain of its components in molten metals gives rise to a variety of unusual effects, the more important being excessive losses from grain boundary areas, transformations in-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Corrosion Terms
An important contribution to corrosion literature has been the publication of more than a hundred definitions of terms frequently used by corrosion engineers. These definitions have been carefully prepared by the Subcommittee on Standard Definitions and Terminology of the Inter-Society Corrosion Committee [Corrosion 14, No. 2, 3 1 ; No. 7, 33 (1958)]. Because persons engaged in some phases of corrosion work may have a different concept of the meaning of a corrosion term than the author, the proper use of these terms by authors and readers would reduce misunderstandings. With the many varied and complex problems which already confront persons engaged in corrosion work, the language barrier certainly should not add to it. Russian Technology
Of possible interest to persons concerned with certain theoretical aspects of aqueous corrosion such as overvoltage, depolarization, and other factors affecting local cell action, is the translation by Levy and Means [Corrosion 14, No. 10, 33 (1958)] of Chapter 4 of the 1945 Russian book, "Theory and Research Methods of Metallic Corrosion" by G. V. Akimov. A more up-to-date view of certain portions of Russian literature on corrosion may be obtained by scanning the titles of papers presented at the" 1958 Ail-Union Scientific-Technical Conference on Corrosion and Protection of Metals held in Moscow in May 1958 [Trans. Elec-
CORROSION
IS A-C POLYETHYLENE
trochem. Soc. 1 0 5 , N o . 1 1 , 230C (1958)].
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Future Outlook S u r f a c e T r e a t m e n t s . Surface i m pregnation treatments wherein several elements a r e diffused i n w a r d simultaneously a n d h a v e a very complex structure, a r e c o m i n g to t h e fore. S u c h metal surfaces m a y h a v e unusually attractive c h a r a c teristics, such as excellent w e a r resistance, unbelievably high h a r d ness, a n d good corrosion resistance. Unverified reports a r e so optimistic t h a t they seem unbelievable. Titanium. T h e price of r a w t i t a n i u m took a n o t h e r p l u n g e d o w n w a r d several m o n t h s a g o . R a w t i t a n i u m sponge, A - l g r a d e , is n o w $1.82 p e r p o u n d ; 10 m o n t h s a g o it w a s $ 2 . 2 5 ; 5 years a g o it w a s $5.00. Stainless steel will c o m p e t e m o r e a n d m o r e w i t h t i t a n i u m for many corrosion-resistant appli cations, as t i t a n i u m emerges from t h e r e a l m of aircraft interest into the b r o a d e r field of general appli cation to A m e r i c a n industry. B e r y l l i u m . Interest m a y b e ex pected t o increase i n t h e corrosion resistance of beryllium, which is b e c o m i n g increasingly i m p o r t a n t for aircraft, missiles, a n d nuclear r e a c tors. N o w t h a t its purity is being increased, its previously believed poor formability is looked a t opti mistically a n d a variety of shapes a n d forms a r e being considered. Beryllium is very light, c o m p a r a b l e to p u r e m a g n e s i u m i n density, b u t of a considerably higher melting point, 2 3 4 0 ° F . (1280° C ) . I t s k n o w n resistance to corrosion b y car b o n dioxide a t elevated t e m p e r a t u r e s , coupled w i t h its very l o w n e u t r o n c a p t u r e cross section, makes it a strong c a n d i d a t e for u s e i n high t e m p e r a t u r e gas-cooled reactors, as fuel " c a n s , " a n d o t h e r core com ponents.
FACTO*
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YOUR PROCESS S t o r i e s of successful a p p l i c a t i o n of A - C P O L Y E T H Y L E N E a r e p o u r i n g i n from so m a n y fields a n d i n d u s t r i e s t h a t w e h a v e n ' t h a d a n o p p o r t u n i t y to explore t h e a p p a r e n t l y limitless uses of this exciting n e w p o l y m e r . S o w e ' r e a s k i n g y o u ! If y o u r field of o p e r a t i o n is even remotely similar t o those below, i t is very p r o b a b l e t h a t A - C P O L Y E T H Y L E N E c a n give y o u i n c r e a s e d sales, lower costs a n d m a n y o t h e r a d v a n t a g e s . F i n d o u t t o d a y !
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When it's
ACmPolyethylene
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