Long-term atmospheric corrosion of Zn–5%Al-coated steel and HDG during outdoor worldwide exposures

Abstract

Hot-dipped galvanised (HDG) steel and Zn5Al-coated steel were exposed for 6 years at different atmospheric weathering sites worldwide. The mass loss of the coatings has been measured after 1, 2, 4 and 6 years of exposure. In addition, corrosion products have been determined using scanning electron microscopy, X-ray diffraction and FTIR spectroscopy. It is shown that Zn5Al is better than HDG-steel independently on the exposure duration and location. The mass loss of Zn5Al is also less dependent of the environmental parameters (such as chloride deposition, relative humidity and temperature) than HDG-steel. The ratio of performance after 6 years of exposure varied from 1.3 to 5.1 with the mean value of 3.1. The analyses of the corrosion products indicate the formation of protective Zn/Al -CO 2− 3 layered double hydroxide (LDH) and sulphate-containing corrosion products of Zn and Al after 6 years of exposure. A mechanism explaining the superior properties of Zn5Al is proposed.

Corrosion mechanisms of Al‐alloyed hot‐dipped zinc coatings in wet supercritical carbon dioxide

“…[ 25 ] In this study, the Z coating showed about 1.2–1.4 times higher corrosion product formation to that of the galfan coating, which is in the lower end of the ratio reported for atmospheric corrosion. [ 46 ] About 2.5 times higher corrosion product formation was obtained for Z compared to that of the galvalume coating. Also, this result correlates with Z and AZ coating reactivities in atmospheric exposure.…”

Corrosion Behavior of Zn-Al-Mg-Si Coatings in Sulfur Dioxide-Containing Environment

“…However, the high S concentration and O content indicate that the SO 4 2− anion has been incorporated into the LDH structure. As previous long-term field exposure studies of ZnAlMg coatings show [17,18], it is possible that SO 4 2− anions may exist in the structure of corrosion products, but they have not been clearly confirmed. However, it is known that LDH compounds easily exchange both cations and anions [26], which, due to the complex composition of the atmosphere in these studies [17,18], may change the identity of interlayer ions.…”

“…As previous long-term field exposure studies of ZnAlMg coatings show [17,18], it is possible that SO 4 2− anions may exist in the structure of corrosion products, but they have not been clearly confirmed. However, it is known that LDH compounds easily exchange both cations and anions [26], which, due to the complex composition of the atmosphere in these studies [17,18], may change the identity of interlayer ions. The presence of two different divalent cations Zn 2+ and Mg 2+ may indicate the coexistence in the corrosion products of the tested coatings of two compounds, Zn-Al SO 4 LDH and Mg-Al SO 4 LDH, in which SO 4 2− anions are incorporated into the layered structure.…”

“…The research provided important information about corrosion products. Corrosion products have been thoroughly characterized in neutral salt spray tests (NSST) [13][14][15], as well as in natural atmospheric environments [5,[16][17][18][19][20][21]. Current data suggest that Zn-Al-Mg coatings and the corrosion products formed thereon provide significant improvements in corrosion resistance in chloride-containing environments.…”

Effect of ZnAl‐LDH on corrosion resistance of Zn–55Al alloy in NaCl solution

“…The phase composition and morphology of the corrosion product film are affected by the alloy composition of the coating, the environmental medium, and the corrosion time. [13,14,[17][18][19][20][21][22][23][24] Corrosion medium can affect the composition of corrosion product film, and the composition of corrosion product film changes with increasing immersion time. However, the difference in composition of the alloy is an important factor that causes the difference in the composition of the corrosion product film.…”