The relations between corrosion rates and different forms of sulfide corrosion product are defined by the application of standard and novel x-ray diffraction studies of the corrosion products. Preliminary studies showed that in aqueous-H2S environments, different sulfides affect the corrosion rate of steel in various ways. But in the absence of an electrolyte none of the sulfides corroded steel appreciably.
More tests were then conducted to relate the product and rate of corrosion, Mild steel coupons were exposed at room temperature to hydrogen sulfide passed through either water, brine or brine with CO2. Product analysis was aided by an improved x-ray technique for identifying very thin corrosion product in place on steel. The initial product of corrosion was found thereby to be a cubic crystal, Fe9S8 (kansite). At first, kansite in the form of tarnish was a source of corrosion resistance, since the corrosion rate decelerated ; then the very thin kansite tarnish grew in all three systems into a thicker kansite scale. The corrosion rate thereon accelerated. In the water system when subsequent scales of pyrrhotite (Fe0.875S) and later pyrite (FeS2) superimposed on kansite scale, the rate again decelerated. In the brine systems the kansite scale only grew thicker; here the rate continued to accelerate.
A proposed explanation for these results is that kansite tarnish consists not of a film but of discrete crystallites, which consolidate into a disjointed, permeable kansite scale. Tarnish photomicrographs give evidence for this view. 3.4.3
