Corrosion performance of Zn–Al–Mg coatings in open and confined zones in conditions simulating automotive applications

Abstract

Panels coated by hot dipping with zinc (HDG), Zn-5Al (Galfan) and Zn-1.5Al-1.5Mg coatings at different thicknesses were phosphated and painted on an industrial line. Crevice panels with non-painted bare parts modelling conditions in hem flanges, reference panels with open surfaces and formed non-painted panels were exposed to a cyclic accelerated automotive test. Zn-Al-Mg coatings with the thickness of 10 mm provided similar or even better protection than HDG and Galfan at 20 mm in both confined and open configurations. In comparison to 10-mm HDG, the Zn-Al-Mg coating delayed red rust appearance in crevices by a factor of 2 and the maximal depth of corrosion in the steel substrate was by 42% lower. Confined areas were more corroded than open surfaces. For HDG, the time to red rust appearance dropped by 50-75%, corrosion attack in steel was from 3.5 to 7 times deeper and mass gain was about 2.3 times higher in crevices than on open surfaces. Corrosion of Zn-Al-Mg may be more affected by local environmental conditions created by the crevice configuration than for HDG. Red rust appearance on formed panels of 20-mm Galfan, 7-, 10-and 14-mm ZnAl-Mg was delayed to 10-mm HDG by a factor of 2.8, 3.5, 3.8 and >4.5, respectively. No adverse effect of forming was noticed. The results indicate that 2-to 3-fold reduction of the coating thickness for Zn-Al-Mg alloy coatings in comparison to traditional HDG may be possible without compromising the corrosion performance.

Accelerated cyclic corrosion tests

“…Zkouška N-VDA a nově vyvinutý test podobný Renault ECC1 D17 2028 simulovaly expozici na atmosférické přímořské stanici nejlépe. Japonští a později evropští a další výrobci oceli vyvíjejí od 90. let 20. století novou generaci zinkových povlaků s přídavkem hořčíku (složení vnější vrstvy typicky Zn-16 hm.% Mg) nebo hliníku a hořčíku (Zn-2 hm.% Al-2 hm.% Mg a další) [8,[10][11][12][13]. Výhodou těchto slitinových povlaků je dobrá svařitelnost, vynikající tvářitelnost, odolnost adhezních spojů a korozní odolnost [11].…”

“…V realističtějších cyklických zkouškách jako např. N-VDA a Renault ECC1 D17 2028 bývá korozní odolnost slitinových povlaků s hliníkem a hořčíkem obvykle 3-10× vyšší než pro HDG [10]. Expoziční zkoušky v reálných atmosférických podmínkách pak ukazují přibližně dvojnásobnou korozní odolnost těchto slitinových povlaků.…”

Microstructure and Corrosion Resistance Property of a Zn-AI-Mg Alloy with Different Solidification Processes

“…Comparing with these coatings, Zn-Al-Mg alloy coating with much higher corrosion resistance in atmosphere and excellent corrosion resistance at cut edge attracted much attention in the last decades [4][5][6][7]. In the 80s last century, Several Zn-Al-Mg coatings composed with 6 wt.% to 11 wt.% Al and 0.5 wt.% to 3.0 wt.% Mg were applied in Japan to prolong life of buildings [8].…”

Thermal stability and Anti-Corrosion analysis of alloy coatings on Overhead Ground Wires

“…Several commercial overhead ground wires for heavy corrosion atmosphere, such as Zn-55Al-0.2Si has been available through 1990s by Japanese and European research groups [11][12][13][14][15][16]. Some researchers found that corrosion inhibited by the buffering and formation of Al(OH) 3 layered in Zn-Al series coatings [17] which attributed to the enhanced corrosion performance of corrosion products in presence of Mg 2+ ions [8,13].…”

“…Some researchers found that corrosion inhibited by the buffering and formation of Al(OH) 3 layered in Zn-Al series coatings [17] which attributed to the enhanced corrosion performance of corrosion products in presence of Mg 2+ ions [8,13]. Corrosion behaviors of alloy coatings in chloride and different anions containing environments have already been investigated [11,14,18]. However, the reason for thermodynamic characteristics is not clear and there is fewer explanations for the proposals of corrosion resistance mechanisms in tests as mentioned above.…”