Iron-chromium alloys pitting corrosion

Description and corrosion resistance. Incoloy 825 is a nickel-iron-chromium alloy with additions of molybdenum and copper. It has excellent resistance to both reducing and oxidizing acids, stress-corrosion cracking, and localized attack such as pitting and crevice corrosion. The alloy is especially resistant to sulfuric and phosphoric acids. 

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. 

The major alloying element contributing to resistance to pitting corrosion in iron- and nickel-base alloys is chromium. The effect of chromium in reducing both the critical current density and the passivating potential of iron in 1 N H2S04 is shown by the polarization curves of... 

While in active range, the alloying elemaits go into solution in their lowest valencies, during corrosion above (/, iron(III) and chromium(VI) compounds are formed. If chlorides are now added to the corrosive medium, an increase in current is observed at Up, this being due to pitting corrosion (Figure 20.21). Chlorides as well as bromides and iodides thus decrease the passive range of the material. 

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 ... 

Stainless steels are alloys of iron to which a minimum of 11% chromium has been added to provide a passive film to resist "rusting" when the material is exposed to weather. This film is self-forming and self-healing in environments where stainless steel is resistant. As more chromium is added to the alloy, improved corrosion resistance results. Consequently, there are stainless steels with chromium contents of 15,17, and 20%, and even higher. Chromium provides resistance to oxidizing environments such as nitric acid and also provides resistance to pitting and crevice attack. 

The properties of the interface metal/solution. Cast iron corrodes because of exposure of its graphite to the surface (graphitic corrosion), which is cathodic to both low-alloy and mild steels. The trim of a valve must always maintain dimensional accuracy and be free of pitting and hence it should stay cathodic to the valve body. Hence, in aggressive media, valve bodies are frequently chosen of steel rather than cast iron. Because of increased anodic polarization, low-alloy steel (Cr and Ni as noble components) is cathodic to normal steel in most natural media. Accordingly, steel bolts and nuts coupled to underground mild steel pipes, or a weld rod used for steel plates on the hull of a ship, should always be of a low-nickel, low chromium steel or from a similar composition to that of the steel pipe.7... 

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. 

Alloys such as 304 and 316 stainless steel or nickel-chromium cdloys exhibit deep pitting in low flow conditions, yet at high seawater velocities their corrosion rate decreases to less than 25 pm per year. Contrary to this, iron and copper show significantly lower corrosion rates at low flow velocities than rmder high seawater flow conditions [37]. 

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