Pitting alloying addition

It is also of interest to note that Wranglen considers that the decrease in the corrosion rate of steel in the atmosphere and the pitting rate in acid and neutral solution brought about by small alloying additions of copper is due to the formation of CU2S, which reduces the activity of the HS and Scions to a very low value so that they do not catalyse anodic dissolution, and a similar mechanism was put forward by Fyfe etal. to explain the corrosion resistance of copper-containing steels when exposed to industrial atmospheres. 

It should be noted that the data refer mostly to the behaviour of the alloys in H2SO4. Passivity is, however, influenced by the composition of the solution as well as that of the metal and for this reason the influence of alloying additions may be different in solutions containing other ions. In particular, CI and other similarly aggressive ions have a large influence and may prevent passivation, either completely or partially. If passivity cannot be maintained over the entire surface of the metal, pitting develops, and this is considered later. 

Molybdenum is widely used as an alloying addition to stainless steels to facilitate the formation of the passive film and to improve resistance to pitting attack (see Section 3.3). 

R.J. Brigham and E.W. Tozer, Effect of Alloying Additions on the Pitting Resistance of 18% Chromium Austenitic Stainless Steel, Corrosion, Vol 30, 1974, p 161-166... 

These measurements lead, in particular, to an understanding that there is a critical solution composition that must be maintained within the pit, and if this does not occur then the pit will die. As a consequence, there is an important effect of the kinetics of active dissolution of the alloy within this critical solution. Measurements of the characteristics of metastable pits, coupled with studies of the dissolution kinetics of steel within the aggressive solutions that characterize the local pit environment, have provided an explanation as to why certain alloy additions (specifically Mo) act to inhibit pitting corrosion by making the maintenance of metastable pits more difficult. Other important effects include those of salt films, which... 

To understand the influence of allo5dng elements on the passivity of stainless steels, researchers have combined electrochemical and siufece analysis. Polarization diagrams provide the first indication of the overall influence of alloy additions on the active-passive transition, passivity, and pitting resistance. However, siuface analysis by X-ray photoelectron spectroscopy (XPS) of prepassivated siufaces provides a direct observation of the location and the chemical state of an alloying element. Such... 

Alloying additions of chromium, nickel, molybdenum, increase the pitting resistance, whereas silicon, sulfur, carbon and nitrogen, decrease the resistance to pitting. Addition of titanium increase resistance of steels only in solutions of FeCl3 and not in other mediums. 

Alloying additions of less than 5% have little effect on seawater corrosion. The susceptibility to pitting is least for 3% Cr-steel than plain carbon steel. Table 9.19 shows the corrosion rates of several low alloy steels containing small quantities of alloying elements (Cu, Ni, Cr) in various combinations. 

TABLE 8.35 Influence of Different Alloying Additions and Microstructure on the Pitting and Crevice Corrosion Resistance of Duplex Stainless Steels... 

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