Corrosion behaviour of hot dip zinc and zinc-aluminium coatings on steel in seawater

A comparative investigation of hot dip Zn-25Al alloy, Zn-55Al-Si and Zn coa tings on steel was performed with attention to their corrosion performance in seawater. The results of 2-year exposure testing of these at Zhoushan te st site are reported here. In tidal and immersion environments, Zn-25Al all oy coating is several times more durable than zinc coating of double thickn ess. At long exposure times, corrosion rate for the Zn-25Al alloy coating r emains indistinguishable from that for the Zn-55Al-Si coating of similar th ickness in tidal zone, and is two to three times lower than the latter in i mmersion zone. The decrease in tensile strength suggested that galvanized a nd Zn-55Al-Si coated steel suffer intense pitting corrosion in immersion zo ne. The electrochemical tests showed that all these coatings provide cathod ic protection to the substrate metal; the galvanic potentials are equal to - 1,050, - 1,025 and - 880 mV (SCE) for zinc, Zn-25Al alloy and Zn-55Al-Si coating, respectively, which are adequate to keep the steel inside the immu nity region. It is believed that the superior performance of the Zn-25Al al loy coating is due to its optimal combination of the uniform corrosion resi stance and pitting corrosion resistance. The inferior corrosion performance by comparison of the Zn coating mainly results from its larger dissolution rate, while the failure of the Zn-55Al-Si coating is probably related to i ts higher susceptibility to pitting corrosion in seawater.