Near-surface intermixing

Ref. [43] for an illustrative and detailed overview. An important observation common for Type II surface alloys is the low onset temperature of near-surface intermixing, that is, of the observable disappearance of guest species from the outermost layer. According to ion scattering spectroscopic data, increasing/decreasing peaks of the host/guest metal are observable just above room temperature for Ni/Pt(lll) [44]. 

Sulfides are intermixed with iron oxides and hydroxides on carbon steels and cast irons. The oxides are also produced in the corrosion process (Reaction 6.6). Although theoretical stoichiometry of 1 to 3 is often suggested between sulfide and ferrous hydroxide, empirically the ratio of iron sulfide to ferrous hydroxide is highly variable. Sulfide decomposes spontaneously upon exposure to moist air. Additionally, corrosion-product stratification is marked, with sulfide concentration being highest near metal surfaces. 

End effects were likely important for Incoloy 800 coupons. Relatively coke—free surface metal) enriched in both chromium and titanium> was visible at the upstream and/or downstream ends of several Incoloy 800 coupons subjected to 0.05 atm. acetylene at 800°C. Numerous small craters> or holes> were observed on the metal. A few isolated filaments protruded from the surface> and sparse amounts of globular coke were also detected at the ends. The bulk of the coke deposit) which consisted of filaments intermixed with larger amounts of globular coke (Figure 8A)> occurred near the midsection of the coupon. A matrix of smooth and rather solid carbon was visible at the metal to coke transition regions the smooth matrix lay under the globular coke. 

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