Materials Performance

MAY 2015

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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60 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 ent water cuts all consisted of two parts, both composed of Fe, O, C, and Ca ele- ments. XRD results, as presented in Figure 6, indicate that these corrosion product scales consisted of FeCO 3 with traces of cal- cium carbonate (CaCO 3 ). The Ca content decreased with the depth of scale. Formation Mechanisms of Corrosion Products on 3Cr and L245 Steels At 10 and 30% water cut, the oil/water mixture emulsion was in the water-in-oil state. At 10% water cut, crude oil covered the steel surface and prevented water from coming in contact with the steel; conse- quently, corrosion was slow. At this condi- tion, the Cr element basically had no effect on the corrosion resistance of steel. How- ever, at 30% water cut, due to the increase of free water, some water covered the sur- face, resulting in an increase of matrix dis- solution. At this condition, L245 steel suf- fered general corrosion, while a Cr-rich layer formed on 3Cr steel, and localized corrosion occurred under the Cr-rich layer. At 80% water cut, the water-oil mixture changed into an oil-in-water emulsion, and the steel matrix surface was completely covered by water. The corrosion of L245 steel was aggravated. It is assumed that the formation of FeCO 3 includes nucleation and growth, 8 with the relative supersatura- tion σ (σ = S–1, where S is the supersatura- tion of FeCO 3 ); and growth predominates at low σ, while nucleation is the control step when σ >> 1. Therefore, when precipitation of FeCO 3 was initiated, σ was larger and nucleation of FeCO 3 produced a dense cor- rosion product scale on L245 steel. After the dense corrosion product scale com- pletely covered the matrix surface, the cor- rosion rate decreased and σ decreased; therefore, the growth of FeCO 3 dominated, and bigger crystals formed on the dense scale surface, forming a porous outer layer. In addition, during the dissolution of 3Cr steel, a Cr-rich layer formed on the matrix surface, and the formation of chro- mium hydroxide [Cr(OH) 3 ] resulted in a lower pH value near the matrix surface. 9 Consequently, FeCO 3 was harder to precipi- FIGURE 2 The morphologies of corrosion product scales on 3Cr steel sample surfaces with (a) 10%, (b) 30%, and (c) 80% water cut. FIGURE 3 The morphologies and compositions analysis of the cross-section of corrosion product scale on 3Cr steel with (a) 10%, (b) 30%, and (c) 80% water cut at 60 °C (A: epoxy, B: scale, C: matrix). MATERIALS SELECTION & DESIGN

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