Materials Performance

MAR 2017

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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53 NACE INTERNATIONAL: VOL. 56, NO. 3 MATERIALS PERFORMANCE MARCH 2017 Corrosion Product Analysis Corrosion products were characterized by x-ray diffraction (XRD), using a Rigaku- D/max 2000 † diffractometer with a Cu K a target under 50 kV, 250 mA, and 2θ = 5- to 90-degrees of range at a scanning speed of 10 degrees/min. Electrochemical Measurements The polarization cur ves were deter- mined using a classic three-electrode cell with platinum as the counter electrode; a saturated calomel electrode (SCE) as the reference electrode; and a noncorroded, Q235 carbon steel sample (yield limit σ s = 235 MPa) with an exposed area of 100 mm 2 as the working electrode. The polarization curves were obtained at a constant scan rate of 10 mV/min. The corrosive electro- lytes were the high-salt water and low-salt water. All the measurements were carried out at room temperature (25 ± 1 °C). Results and Discussion Visual Observation and Water Quality Analysis The family heating system consists of a coal-fired heating unit, a make-up water line, and a delivery side that includes floor- heating pipes or radiators. Figure 1 shows the heating unit. Close observation of the system hardware indicated the presence of severe corrosion. The steel fitting was cor- roded and covered with brown corrosion product. In addition, there was localized corrosion at the weld. A copper valve was also corroded and covered with a greenish- blue copper patina. Figure 2 shows corrosion of the heat supply end, which is typical for each room. The rectangular element in the center of Figure 2(a) is translucent plastic, and rust stains on the inside can be seen. Figure 2(a) also shows deliver y water that is turbid with rust, indicating the deliver y water pipe had suffered from severe corrosion at- tack. Additionally, Figure 2(a) indicates that the delivery water steel pipe outlet and steel fitting had suffered from corrosion. Figures 2(b) and (c) also indicate corrosion attack of the steel fittings in the heat supply FIGURE 1 The heating unit of the coal-fired domestic heating system showing corrosion of the steel fittings and copper valve. system that created yellow-brown or dark- brown corrosion products. For this newly installed domestic heat- ing system, neither deaeration nor soften- ing were used for the delivery and make-up water. The main factors affecting the heat- ing system include water temperature, oxygen concentration, pH value, carbon dioxide (CO 2 ) concentration, and corrosive ions in the water. Water Temperature It was reported that corrosion rates for steel pipe containing water increase with increasing water temperature when other factors remain constant. When the water temperature is <85 °C, every 10 °C increase in the temperature can bring a 30% in- crease in the corrosion rate. 4 Oxygen con- centration in the water, however, decreases as the water temperature increases, and lower oxygen concentration usually results in lower corrosion rates for steel. There- fore, the net effect of water temperature on the steel pipe corrosion in an open system is uncertain. Oxygen Concentration The effect of DO in water that causes the steel corrosion lies in the reduction of

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