海水飞溅区Ni-Cu-P钢的锈层和耐点蚀性能研究

STUDY ON RUST LAYERS AND PITTING CORROSION RESISTANCE OF Ni-Cu-P STEEL EXPOSED IN MARINE SPLASH ZONE

在海水飞溅区对实验室冶炼的Ni-Cu-P钢、含Cu低合金钢和碳钢进行660 d的挂片实验,评价Ni-Cu-P钢的耐蚀性能;采用Fourier变换红外(FTIR)光谱、电感耦合等离子体原子发射光谱(ICP-AES)、电子探针(EPMA)、SEM和EDAX等技术,分析3种钢表面的锈层特征.结果表明,Ni Cu P钢表现出比碳钢优越的耐全面腐蚀和点蚀能力.对锈层成分分析发现,在宏观阴极区,钢的内、外锈层均主要由α-FeOOH,β-FeOOH,γ-FeOOH,δ-FeOOH,Fe_3O_4和少量非晶氧化物组成,但内锈层的Fe_3O_4含量更高,而γ-FeOOH和β-FeOOH的含量更低.与碳钢相比,Ni-Cu-P钢宏观阴极区和蚀坑内的锈层更致密.对锈层中的合金元素分析发现,Ni-Cu-P钢中的合金元素Ni,Cu和P主要分布在宏观阴极区的内锈层和蚀坑内,Cu和P在蚀坑内有富集.在宏观阴极区,合金元素Cu可细化内锈层的晶粒,从而促进保护性锈层的形成.在蚀坑内,Cu富集在锈层中的夹杂物周围,对锈层中的裂纹和孔洞起修复作用.合金元素Cu和Ni可提高蚀坑内基体的电位,P有助于降低钢蚀坑内基体的腐蚀速度,因此,Ni-Cu-P钢比碳钢表现出强的耐点蚀性能.

Ni Cu-P steel is well known as a seawater corrosion resistance steel due to strong corrosion resistance in marine splash zone. However, corrosion resistance mechanisms of alloying elements in Ni Cu P steel remain unclear. Because the steel exhibits obvious characteristic of pitting corrosion in marine splash zone, rust layers and pitting corrosion resistance were investigated in this study. The experimental steels were smelted in vacuum induction melting furnace. In order to evaluate the corrosion resistance of Ni-Cu P steel, hanging plate test was performed in marine splash zone for 660 d. Rust layers formed on the steel surfaces were studied by means of scanning electro microscopy （SEM）, energy dispersive analysis of X ray （EDAX）, Fourier transform infrared resonance （FTIR） and inductively coupled plasma atomic emission spectrometry （ICP AES）. The results indicated that average corrosion rate and pit penetration of Ni Cu-P steel was obviously smaller than that of carbon steel after exposure test. For all the steels, the inner and outer rust layers were composed of ^-FeOOH, /3 FeOOH, ~ FeOOH, ~ FeOOH, Fe304 and a small amount of amorphous oxides. However, the inner rust layer exhibited higher content of Fe304 and lower content of ~ FeOOH and /3-FeOOH than the outer rust layer. Under the same condition, the rust layers both in macro cathodic region and pits of Ni Cu P steel were much more compact than those of carbon steel. According to the composition and distribution of alloying elements, Ni, Cu and P were mainly observed in the inner rust layer and pits, and Cu and P were found to enrich in pits. In macro cathodic region, alloying element Cu made inner rust grains small and dense.In corrosive pits, Cu wa~ observed to enrlcn arounod inclusions in the rust layer,which could repair and fill the slots and holes of the rust layer in pits. Additionally, addition of alloying elements Cu and Ni improved potential of matrix in pits, and alloy element P led to a decrease in the corrosion rate of matrix. Therefore, Ni-Cu-P steel exhibited stronger pitting corrosion resistance than carbon steel.