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(一)在大气中的腐蚀对钢铁材料来说,高湿度、工业区的 SO_2气、沿海地区的 NaCl 增大了大气中的腐蚀,而农业区大气中的腐蚀最小。在图53上表示了5个不同的场所测得的各种钢铁材料的平均腐蚀速度。对所有钢铁材料而言,腐蚀速度最初是大的,随时间的延长逐渐减小。腐蚀速度随时间而减小的原因是在表面生成的腐蚀产物具有保护作用。铸铁具有比钢好的耐蚀性,可认为残留在腐蚀产物中的石墨及比钢高的含硅量形成硅酸铁而显现了保护作用。
(I) Corrosion in the atmosphere For steel materials, high humidity, SO 2 gas in industrial areas and NaCl in coastal areas increase the corrosion in the atmosphere, while the corrosion in the agricultural area is minimal. Figure 53 shows the average corrosion rates of various steel materials measured at five different locations. For all steel materials, the rate of corrosion is large initially and decreases with time. The reason why the corrosion rate decreases with time is that the corrosion products generated on the surface have a protective effect. Cast iron has better corrosion resistance than steel, which can be considered as the residual graphite in the corrosion product and the silicon content higher than that of the steel to form the iron silicate.