Molybdenum corrosion behaviour

The basic corrosion behaviour of stainless steels is dependent upon the type and quantity of alloying. Chromium is the universally present element but nickel, molybdenum, copper, nitrogen, vanadium, tungsten, titanium and niobium are also used for a variety of reasons. However, all elements can affect metallurgy, and thus mechanical and physical properties, so sometimes desirable corrosion resisting aspects may involve acceptance of less than ideal mechanical properties and vice versa. 

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.)... 

The theoretical aspects of molybdenum s corrosion behaviour are complex and there is as yet no clear cut, generally applicable picture. There are, however, a large number of literature references which include data on polarisation, passivation and potential of molybdenum under widely assorted conditions. The electrode potential of molybdenum depends on its surface condition. For example, some tests showed an of -t-0-66V when the molybdenum was passivated by treatment with concentrated chromic acid and —0-74 V after activation by cathodic treatment in sodium hydroxide. 

The US Bureau of Mines found the chemical and galvanic corrosion behaviour of both the TZM and Mo-30W alloy to be generally equal or superior to that of unalloyed molybdenum in many aqueous solutions of acids, bases and salts. Notable exceptions occurred in 6-1 % nitric acid where both alloys corroded appreciably faster than molybdenum. In mercuric chloride solutions the TZM alloy was susceptible to a type of crevice corrosion which was not due to differential aeration. The alloys were usually not adversely affected by contact with dissimilar metals in galvanic couple experiments, but the dissimilar metals sometimes corroded galvanically. Both alloys were resistant to synthetic sea water spray at 60°C. 

Table 5.26 The corrosion behaviour of tantalum-molybdenum alloys in concentrated sulphuric and hydrochloric acids at 55°C solutions saturated with oxygen...

These steels are passive in seawater and show practically no surface corrosion. The pitting corrosion behaviour of steels with molybdenum contents of 2-3% is also much better than in the molybdenum-free austenitic steels. Whereas pitting rates in the range of 0.65-1.58 mm/a (25.6-62.2 mpy) were measured in the molybdenum-free steel SAE 304 (X5CrNil8-10) in slowly flowing seawater, the value for the steel SAE 316 with approx. 2.5% Mo (X5CrNiMol7-12-2) is 0.03-0.34 mm/a (1.18-13.4 mpy) [150]. 

On the corrosion behaviour of austenitic chromium-nickel-(molybdenum) steels with and without addition of nitrogen with special consideration of their strain in chloride containing solutions)... 

Crevice corrosion behaviour of molybdenum-containing stainless steels in seawater... 

Unalloyed cast iron (grey cast iron) exhibits similar behaviour to unalloyed steel in waste water treatment plants. Waste water pumps made of grey cast iron (GG 22, GG 25, 0.6025) have a rough surface that is an ideal substrate for adhesive biofilms that can cause biocorrosion. Biocorrosion cannot be completely excluded on polished cast parts containing spheroidal graphite and silicon as well as on high-alloy CrNiMo cast pumps. The corrosion properties of hard chromium-molybdenum cast iron G-X 250 CrMo 15 3 (cf. G-X 300 CrMo 15 3, 0.9635) are comparable to those of unalloyed cast iron however, the former is a superior material for immersion... 

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