Ferrous alloys corrosion

The non-ferrous alloys include the misleadingly named nickel silver (or German silver) which contains 10-30% Ni, 55-65% Cu and the rest Zn when electroplated with silver (electroplated nickel silver) it is familiar as EPNS tableware. Monel (68% Ni, 32% Cu, traces of Mn and Fe) is used in apparatus for handling corrosive materials such as F2 cupro-nickels (up to 80% Cu) are used for silver coinage Nichrome (60% Ni, 40% Cr), which has a very small temperature coefficient of electrical resistance, and Invar, which has a very small coefficient of expansion are other well-known Ni alloys. Electroplated nickel is an ideal undercoat for electroplated chromium, and smaller amounts of nickel are used as catalysts in the hydrogenation of unsaturated vegetable oils and in storage batteries such as the Ni/Fe batteries. 

B causc ol llicir comxsion rcsisiance and the- I atl ihal copper alloys have heen used for many thoiisaiids of years, the copper alloys do not have the hi h-slreiigth qualities of the ferrous alloys, while their density is comparahle. The cost per bv soldering, which is not shared bv other metals that have reasonable corrosion resistance,... 

Dzhambasova, L., Mikhovskii, M. and Ivanov, D., Acoustic Study of Intergranular Corrosion in Non-ferrous Alloys , Tekh. Misul., 10, 115 (1973)... 

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Little, M. V., Bonding Aluminium To Ferrous Alloys , Machinery, N. Y., 56, 173 (1950) Drewett, R., Diffusion Coatings for the Protection of Iron and Steel , Part I, Anti-Corrosion, 16 No. 4, 11-16, April (1969)... 

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

Rusting (rust) corrosion of iron or ferrous alloys resulting in a corrosion product which consists largely of hydrous ferric oxide. 

Adequate for use in munitions. Polymerizes slowly. Corrosive to ferrous alloys beginning at 65°C. 

Iron and the Ferrous Alloys. Iron exposed to a moist environment reacts with water and atmospheric oxygen to form rust, a brown, crumbly corrosion product (composed of hydrated iron oxide). Initially rust forms a surface layer that is usually held in much disfavor. If an ancient iron object... 

Corrosion is a mixed-electrode process in which parts of the surface act as cathodes, reducing oxygen to water, and other parts act as anodes, with metal dissolution the main reaction. As is well known, iron and ferrous alloys do not dissolve readily even though thermodynamically they would be expected to, The reason is that in the range of mixed potentials normally encountered, iron in neutral or slightly acidic or basic solutions passivates, that is it forms a layer of oxide or oxyhydroxide that inhibits further corrosion. 

A continuous stirred tank reactor is being used to accomplish a second order reaction that is catalyzed by hydrogen ions. Residence time is 0.2 hrs. Under normal conditions the inlet acid concentration is 0.002 N. The tank is made partly of ferrous alloy that corrodes slowly in the acid environment. In contact with 0.001 N acid, laboratory results show that the corrosion rate is... 

Hastelloy C has superb corrosion resistance over wide ranges of pH and Eh- The main disadvantage of such non-ferrous alloys over stainless steels is usually cost, although the vulnerability of high-nickel and copper alloys to attack by chelating agents (Sections 14.4.1 and 16.5) should be borne in mind. 

When a ferrous alloy is immersed in phosphoric acid, il initially forms a soluble phosphate. As the pH rises at the mclal/solutiun interface, the phosphate becomes insoluble and crystallizes epitaxially on Ihe substrate metal. The phosphate coating thus produced consists of a nonconduciivc layer nf crysinlx that insulates the metal from any subsequently applied film and provides a topography with enhanced tooth" for increased adhesion. The cry stals insulate microanode and microcathode centers caused by stress or imperfections in the metal surface. This greatly reduces Ihe severity of electrochemical corrosion. 

Caustic embrittlement is the development of brittleness iit metals such as steel or ferrous alloys, upon prolonged exposure to alkaline substances, like caustic soda, in solution. Failures and explosions in boilers and evaporators have been caused by this action. Effective water treatment essentially has eliminated this condition in boilers. See also Corrosion Embrittlement. 

Chromium is used in the manufacture of stainless steel and other specialist steels, and non-ferrous alloys. Chromate salts are used as tanning agents, pigments, catalysts, corrosion inhibitors and in electroplating solutions. Although stainless steel is inert, the dissolution of chromium from this steel is likely to be the major source of chromium in food. 

M. W. Kendig and R. G. Buchheit. Surface Conversion of Aluminum and Ferrous Alloys for Corrosion Resistance. In Proceedings of the Research Topical Symposium CORROSION/2000. 2000 NACE International. 

A more plausible theory could be a combination of the two previous approaches with the relative importance of one or the other depending on the particular system, and is therefore a function of the medium, surface finishing and the nature of materials in contact. However, in both approaches the presence of oxygen accelerates the corrosion by fretting, especially for the ferrous alloys.16... 

Rebak, R.B., Corrosion of Non-Ferrous Alloys, Part I Nickel-, Cobalt-, Copper-, Zirconium-and Titanium-Base Alloys, Corrosion and Environmental Degradation, Vol. II, Wiley-VCH, Weinheim, p. 69, 2000. 

The development of CVD films for high-temperature corrosion protection of ferrous alloy has focused on methods with and without addition of oxygen-active elements. CVD processes, which employ vapor-transport and heat treatment of a stabilized alloy substrate to... 

Next we consider the Pourbaix diagram for iron, which is, of course, of paramount importance for the understanding of corrosion of ferrous alloys such as the many types of steel and stainless steel. This is a rather complex diagram, since two oxidation states of iron exist both in the liquid and the solid state and the metal is amphoteric to some extent. Figure 16M is a simplified version of the diagrams shown in the original work of Pourbaix. The two soluble species in acid solutions are Fe and Fe. The relevant equilibria are... 

A similar reaction occurs during pitting corrosion of iron and its alloys. Partial hydrolysis, leading to the formation of Al(OH) and Al(OH) may also occur, but all such reactions lead to the formation of acid, making the solution inside the pit much more aggressive than outside. Measurement of the pH inside a pit is not an easy matter, but estimates based on various calculations and on measurements in model pits lead to values as low as 1-2 for chromium-containing ferrous alloys and about 3.5 for aluminum-based alloys, depending on experimental conditions. 

Hydrochloric add Nitric add Dissolves most water scales and corrosion products Dissolves most water scales and corrosion products On boilers, heat exchangers, pipelines, etc. On stainless steel and aluminum Corrosive to steel temperature must be below 175T Cannot be used on copper and ferrous alloys... 

---