The Corrosion Behavior Of Hot Dip Zinc And Its Alloys In Seawater

Hot-dip zinc and alloy are widely used in all over the world. The research about hot-dip steels in all kinds of atmosphere environment is quite systemic and in-depth, however, the research on corrosion behavior and corrosion mechanism in seawater is relatively less, especial the anti-corrosion research about different layers of coating. The anti-corrosion research in seawater of several famous hot-dip coatings was comparative in this paper, and the corrosion behaviors of different layers in seawater were discussed on emphasis of coating structure. The corrosion of different layers was analyzed by the method of polarization-relaxation-EIS measure, which was founded by ourselves. The corrosion characters and process of different corrosion phases were estimated by research of EIS characters and change of electrochemistry parameters. The corrosion models of four kinds of coatings were put forward respectively. The corrosion of all hot-dip coating steel wire was divided into several phases, and the corrosion of each phase was corresponded to different layer respectively. The anti-corrosion difference of every layer was very distinct. The optimal anti-corrosion layer of Zn-5%Al-0.2%RE was located between eutectic layer and intermetallic layer; the optimal anti-corrosion layer of Zn-55%Al-1.6%Si was intermetallic layer (Al-Si-Fe); the optimal anti-corrosion layer of Zn-0.2%Al-RE was compact area of δ phase, and the optimal anti-corrosion layer of Zn was δ phase. When local breakage appears, coating is accelerated dissolved as anode, which provides electrochemistry protection on steel. However, the research on this protection is only qualitative heretofore. In this paper, the electrochemistry protection of several hot-dip coatings was quantitatively estimated referring to correlative standard of sacrificial anode. The results showed that, at the same temperature, the open-circuit potential and work potential were positive move with the Al content increase of alloy anode, and the electric current efficiency decreased, the corrosion uniformity was even worse, and the electrochemistry protection declined. For the same alloy anode, the open-circuit potential and work potential were positive move with improve of temperature, while the electric current efficiency and the electrochemistry protection decreased. Zn-55%Al-1.6%Si has advantage over Zn-5%Al-0.2%RE on protecting the border of coating steel, which can provide longer protection. It is distinctly superior for Zn-55%Al-1.6%Si on resisting the corrosion of salt fog than Zn-5%Al- 0.2%RE. The corrosion rate of Zn-5%Al-0.2%Re and Zn-55%Al-1.6%Si in room simulant immersed experiment is obviously smaller than the corrosion rate of Zn-0.2%Al-RE and Zn. The corrosion rate of four kinds of samples is fastest at the start, then decreases distinctly. The galvanic corrosion experiments of Zn-5%Al-0.2%RE with carbon steel and Zn-55%Al-1.6%Si with carbon steel were carried thorough respectively on basis of condition in which polarity reversal of Zn-Fe couple or Al-Fe couple was observed, and the polarity reversal was not observed at this condition.