Chromium-molybdenum alloys pitting corrosion

Nickel-chromium-molybdenum alloys are used in reactor vessels in the production of acetic acid. These alloys are cost-effective compared to Ni-Cr stainless steels and have good resistance to oxidizing corrosive media Ni-Mo alloys have good resistance to reducing media. Molybdenum together with the chromium stabilizes the passive film in the presence of chlorides and is particularly effective in increasing resistance to pitting and crevice corrosion. 

Water environments can also have a variety of compositions and corrosion characteristics. Freshwater normally contains dissolved oxygen as well as minerals, several of which account for hardness. Seawater contains approximately 3.5% salt (predominantly sodium chloride), as well as some minerals and organic matter. Seawater is generally more corrosive than freshwater, frequently producing pitting and crevice corrosion. Cast iron, steel, aluminum, copper, brass, and some stainless steels are generally suitable for freshwater use, whereas titanium, brass, some bronzes, copper-nickel alloys, and nickel-chromium-molybdenum alloys are highly corrosion resistant in seawater. 

Molybdenum improves the corrosion resistance of stainless steels that are alloyed with 17—29% chromium. The addition of 1—4% molybdenum results in high resistance to pitting in corrosive environments, such as those found in pulp (qv) and paper (qv) processing (33), as weU as in food preparation, petrochemical, and poUution control systems. 

Nickel is usually alloyed with elements including copper, chromium, molybdenum and then for strengthening and to improve corrosion resistance for specific applications. Nickel-copper alloys (and copper-nickel alloys see Section 53.5.4) are widely used for handling water. Pumps and valve bodies for fresh water, seawater and mildly acidic alkaline conditions are made from cast Ni-30% Cu type alloys. The wrought material is used for shafts and stems. In seawater contaminated with sulfide, these alloys are subject to pitting and corrosion fatigue. Ammonia contamination creates corrosion problems as for commercially pure nickel. 

The effect of the chromium content of the alloy on corrosion in boiling acids is shown in Table 4.7 along with the data for carbon steel and low-carbon and low-nitrogen 35% Cr alloys. The data show that the corrosion rates of 18 Cr-8 Ni (Type 304) is lower than Type 430 and 446 that is devoid of nickel. The nickel is the alloy probably reduces the rate of hydrogen evolution reaction. The molybdenum in Type 316 alloy was found to be useful in protection from pitting by chloride ions. 

Pitting resistance equivalent number (PREN) is derived from a formula involving chromium, molybdenum and nitrogen contents of the alloys. These numbers are used as a qualitative indication of the pitting resistance of the alloys. The higher the PREN value the greater the resistance of the alloys to corrosion due to chloride. 

The addition of molybdenum to the alloy, as in type 316, increases the corrosion resistance and high-temperature strength. If nickel is not included, the low-temperature brittleness of the material is increased and the ductility and pit-type corrosion resistance are reduced. The presence of chromium in the alloy gives resistance to oxidizing agents. Thus, type 430, which contains chromium but no nickel or molybdenum, exhibits excellent corrosion resistance to nitric acid and other oxidizing agents. 

The main approach to improving the pitting and crevice corrosion resistance of the basic 35% nickel, 19% chromium, and 2% molybdenum alloy was to increase the molybdenum content. Among the first of the newer alloys introduced was 904 L (UNS N08904), which boosted the molybdenum content to 4% and reduced the nickel content to 25%. The reduction in nickel content was beneficial as a costsaving factor, with minimal loss of general corrosion resistance and sufficient resistance to chloride SCC. 

By alloying nickel with both molybdenum and chromium, an alloy is obtained resistant to oxidizing media imparted by alloyed chromium, as well as to reducing media imparted by molybdenum. One such alloy, which also contains a few percent iron and tungsten (AUoy C), is immune to pitting and crevice corrosion in seawater (10-year exposure) and does not tarnish appreciably when exposed to marine atmospheres. Alloys of this kind, however, despite improved resistance to Cl, corrode more rapidly in hydrochloric acid than do the nickel-molybdenum alloys that do not contain chromium. 

The chromium-free nickel-molybdenum alloys, for example NiMo28 (DIN-Mat. No. 2.4617, HasteDoy B-2), show only minimal amounts of surface corrosion in seawater, but are nonetheless practically unsuitable for use in seawater because of their sensitivity to pitting and crevice corrosion. 

Among the alloying elements used to improve the corrosion resistance of passivated alloys, molybdenum plays a central role in stainless steels. Indeed, stainless steels (iron-chromium or iron-chromium-nickel alloys) that contain molybdenum offer much better corrosion resistance (especially against pitting) than those without molybdenum. Despite the enormous amount of research work carried out on the process involved, using surface analytical methods combined with electrochemical measurements, the exact mechanism of the effect of molybdenum is not fully understood, and is still a matter of debate. However, all the data indicate that the improved corrosion resistance brought about by alloyed molybdenum is due to different phenomena, which may be rationalized in the following way ... 

This is a chromium-nickel-molybdenum alloy, with its composition shown in Table 8.4. It has excellent resistance to chloride pitting and stress corrosion cracking environments. It finds use in the chemical processing and utility industries. 

Chromium, molybdenum, and nitrogen are the most efficient alloying elements for increasing the resistance to crevice (and to pitting) corrosion initiation of stainless alloys (Fe-Ni-Cr-Mo-N-... alloys). A pitting index has been derived that characterizes the resistance of stainless alloys to these forms of corrosion [14] (Fig. 12 [15]) ... 

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