Chromium corrosion

Stainless steels contain 11% or more chromium. Table 5.1 lists common commercial grades and compositions of stainless steels. It is chromium that imparts the stainless character to steel. Oxygen combines with chromium and iron to form a highly adherent and protective oxide film. If the film is ruptured in certain oxidizing environments, it rapidly heals with no substantial corrosion. This film does not readily form until at least 11% chromium is dissolved in the alloy. Below 11% chromium, corrosion resistance to oxygenated water is almost the same as in unalloyed iron. 

Chromium is usually present as green-black chromium trioxide and is formed as a result of the reduction of hexavalent chromium corrosion inhibitor. 

Lee S-H, Brennan FR, Jacobs JJ, et al. 1997. Human monocyte/macrophage response to cobalt-chromium corrosion products and titanium particles in patients with total joint replacements. J Orthop Res 15 40-49. 

Chromium Corrosion and oxidation protection of metal parts. 

In the scrubbing section of the HA column, fission products were scrubbed from the organic phase leaving the extraction section by the HAS scrub stream. It contained 0.01 Af H3PO4 to complex protactinium and zirconium-niobium and reduce their extraction. It also contained 0.01 Af ferrous sulfamate to reduce plutonium and chromium corrosion product to inextractable species. 

Alloy Steels. These contain a iittie carbon, and sometimes siiicon, but they mainiy contain added metais, such as manganese (hardness), nickei (strength), moiybdenum (improved wear), tungsten (high temperature strength), chromium (corrosion resistance), and vanadium (toughness). 

Fig. 5.18—cont d (a) Tafel polarization curves for chromium corrosion diagram showing the corrosion potential and the corrosion current, (b) The corrosion current and the potential if mass limitations for hydrogen limit the maximum current to 10 A/cm, and (c) to 10 A/cm, respectively. 

It has been shown [74] that this condenser results in lower corrosion rates than the Allihn condenser. The lowest rates are obtained with (glass) apparatus in which there is continuous flushing of (chromium) corrosion products [6,74]. A design for such conditions has also been provided by Corbett [38]. In this apparatus, up to 100 specimens can be tested simultaneously with corrosion products kept so low that the test simulates a once through industrial application. 

This is a high-carbon, high-chromium, corrosion-resistant alloy that can be described as either a high-hardness type 440C or a corrosion resistant, D2 tool steel. It possesses corrosion resistance equivalent to type 440C stainless but can attain a maximum hardness of Rockwell C 64, approaching that of tool steel. 

This reaction is of significance mainly in the case of alloys containing relatively large amounts of chromium. Corrosion proceeds by the selective oxidation of Cr at the hotter loop surfaces and reduction and deposition of chromium at the cooler loop surfaces. In some solvents (Li,Na,K,U/P for example) the equilibrium constant for reaction (5.9) with Cr changes sufficiently as a function of temperature to cause formation of dendritic chromium crystals in the cold zone. Por Li,Be,U/P mixtures the temperature dependence of the mass transfer reaction is small, and the equilibrium is satisfied at reactor temperature conditions without the formation of crystalline chromium. Of course, in the case of a coolant salt with no fuel component, reaction (5.9) would not be a factor. 

In Russia, materials testing for Th-U MSR was started atNRC-KIin 1976 [15,16,43]. It was substantiated by available experience accumulated in ORNL MSR program on nickel -base alloys for UF4-containing salts. The Ni-based alloy HN80MT was chosen as a base (see Table 5.3). Its composition (in wt.%) is Ni - 7,0Cr - 0.04C - l.TTi -I2.IM0. The development and optimization of HN80MT alloy was envisaged to be performed in two directions, including (1) improvement of the alloy resistance to a selective chromium corrosion and (2) increase of the alloy resistance to tellurium IGC. 

Chromium is used to harden steel, to manufacture stainless steel, and to form many useful alloys. Much is used in plating to produce a hard, beautiful surface and to prevent corrosion. Chromium gives glass an emerald green color and is widely used as a catalyst. 

Common alloying elements include nickel to improve low temperature mechanical properties chromium, molybdenum, and vanadium to improve elevated-temperature properties and silicon to improve properties at ordinary temperatures. Low alloy steels ate not used where corrosion is a prime factor and are usually considered separately from stainless steels. 

Chromium is the most effective addition to improve the resistance of steels to corrosion and oxidation at elevated temperatures, and the chromium—molybdenum steels are an important class of alloys for use in steam (qv) power plants, petroleum (qv) refineries, and chemical-process equipment. The chromium content in these steels varies from 0.5 to 10%. As a group, the low carbon chromium—molybdenum steels have similar creep—mpture strengths, regardless of the chromium content, but corrosion and oxidation resistance increase progressively with chromium content. 

Standard Wrought Steels. Steels containing 11% and more of chromium are classed as stainless steels. The prime characteristics are corrosion and oxidation resistance, which increase as the chromium content is increased. Three groups of wrought stainless steels, series 200, 300, and 400, have composition limits that have been standardized by the American Iron and Steel Institute (AlSl) (see Steel). Figure 8 compares the creep—mpture strengths of the standard austenitic stainless steels that are most commonly used at elevated temperatures (35). Compositions of these steels are Hsted in Table 3. 

Ferritic stainless steels depend on chromium for high temperature corrosion resistance. A Cr202 scale may form on an alloy above 600°C when the chromium content is ca 13 wt % (36,37). This scale has excellent protective properties and occurs iu the form of a very thin layer containing up to 2 wt % iron. At chromium contents above 19 wt % the metal loss owiag to oxidation at 950°C is quite small. Such alloys also are quite resistant to attack by water vapor at 600°C (38). Isothermal oxidation resistance for some ferritic stainless steels has been reported after 10,000 h at 815°C (39). Grades 410 and 430, with 11.5—13.5 wt % Cr and 14—18 wt % Cr, respectively, behaved significandy better than type 409 which has a chromium content of 11 wt %. 

AISI 321 and 347 are stainless steels that contain titanium and niobium iu order to stabilize the carbides (qv). These metals prevent iatergranular precipitation of carbides during service above 480°C, which can otherwise render the stainless steels susceptible to iatergranular corrosion. Grades such as AISI 316 and 317 contain 2—4% of molybdenum, which iacreases their creep—mpture strength appreciably. In the AISI 200 series, chromium—manganese austenitic stainless steels the nickel content is reduced iu comparison to the AISI 300 series. 

Anodes. Lead—antimony (6—10 wt %) alloys containing 0.5—1.0 wt % arsenic have been used widely as anodes in copper, nickel, and chromium electrowinning and metal plating processes. Lead—antimony anodes have high strength and develop a corrosion-resistant protective layer of lead dioxide during use. Lead—antimony anodes are resistant to passivation when the current is frequendy intermpted. 

---