High sulphidation rate

Various biologieal faetors affeet eorrosion in soils. Organic acids originating from humus are relatively eorrosive versus steel, zinc, lead and copper. In certain soil types, particularly acidic clay, very high corrosion rates have been experieneed under anaerobie eonditions. This is commonly caused by sulphate-reducing bacteria, which is indicated by ferrous sulphide found as part of the corrosion products. The mechanism of this type of eorrosion has been dealt with in Section 6.4. 

The reaction of metals with gas mixtures such as CO/CO2 and SO2/O2 can lead to products in which the reaction of the oxygen potential in the gas mixture to form tire metal oxides is accompanied by the formation of carbon solutions or carbides in tire hrst case, and sulphide or sulphates in the second mixture. Since the most importairt aspects of this subject relate to tire performairce of materials in high temperature service, tire reactions are refeiTed to as hot corrosion reactions. These reactions frequendy result in the formation of a liquid as an intermediate phase, but are included here because dre solid products are usually rate-determining in dre coiTosion reactions. 

Mrowec et examined the resistance to high-temperature corrosion of Fe alloys with Cr contents between 0.35 and 74 at% Cr in 101 kPa S vapour. They found that the corrosion was parabolic, irrespective of the temperature or alloy composition, and noted that sulphidation takes place at a rate five orders of magnitude greater than oxidation at equivalent temperatures. At less than 2% Cr, the alloys formed Fe, j.,S growing by outward diffusion of Fe ions, with traces of FeCr2S4 near the metal core. 

The sulphide usually forms an interconnected network of particles within a matrix of oxide and thus provides paths for rapid diffusion of nickel to the interface with the gas. At high temperatures, when the liquid Ni-S phase is stable, a duplex scale forms with an inner region of sulphide and an outer porous NiO layer. The temperature dependence of the reaction is complex and is a function of gas pressure as indicated in Fig. 7.40 . A strong dependence on gas pressure is observed and, at the higher partial pressures, a maximum in the rate occurs at about 600°C corresponding to the point at which NiS04 becomes unstable. Further increases in temperature lead to the exclusive formation of NiO and a large decrease in the rate of the reaction, due to the fact that NijSj becomes unstable above about 806°C. 

In the Salton Sea area, California, silica in the hyper-saline brine (Table 3) has been removed from solution in special brine clarification tanks. A sludge containing precipitates of silica and various sulphides is injected into hot brine in the tanks. The precipitates in the sludge act as nuclei for precipitation of additional silica and the rate of precipitation is sufficiently high for removal of aqueous silica to < 100 ppm. This method most likely only applies to very saline waters. 

Deposition by a pure ion-by-ion mechanism should also solve this problem, since no hydroxide is involved. However, in this case we encounter the problem of the high K p of ZnS compared to CdS, which again means that more sulphide is needed. For thiourea, this means a higher pH, which again means that strong com-plexation is needed to prevent Zn(OH)2 formation, by reducing the free [Cd ]. However, this will also reduce the rate of ZnS deposition. While there are many examples in the literature of cluster deposition of ZnS, there does not even seem to be one unambiguous case of ion-by-ion deposition of this semiconductor. 

Low sulphur stocks have better ageing resistance. Normally the oxidation rate increases with the amount of sulphur used in the cure. The increased rate may be due to the activation of adjacent C-H groups by high levels of combined sulphur. Saturated sulphides are inert to oxidation. 

The first reports on the incorporation of molybdenum disulphide in a metal matrix were those mentioned previously which were published by Bowden in 1950. He reported a coefficient of friction of 0.13 for a composite in sintered copper. In his other metallic composite the molybdenum disulphide was formed in situ by hydrogen sulphide in sintered molybdenum and had a coefficient of friction of 0.06. At about the same time R L Johnson et al at NACA studied the effect of molybdenum disulphide concentration in silver with 5% of copper. They reported coefficients of friction as low as 0.17 and found that the friction decreased with increasing concentration of molybdenum disulphide. Their wear rates were high, around 10 mm /Nm, but this work was the fore-runner of many studies using the same components. 

---