Evaluation of surface activity of hot-dip galvanized steel after alkaline cleaning

Multiscale and Multi-Technical Approach to Characterize the Hot-Dip Galvanized Steel Surface and Its Consequence(s) on Paint Adhesion and Tendency to Blistering

“…Due to a small addition of aluminum (0.1-0.3 wt%) in the galvanizing bath, to its limited solubility in zinc matrix, and to its affinity to oxygen, the surface of hot-dip galvanized steel (HDG) is enriched in aluminum. An atomic concentration ratio of Al to Zn of about 1-3:1 for the uppermost 30 Å was observed by Feliu et al [1] on HDG and Galfan (Zn-5Al) surfaces and the presence of a thin film of aluminum oxide at the extreme surface is now well accepted and documented [1][2][3][4][5][6][7][8][9].…”

“…One should note that although the pH was varied, the same chemistry was used for the cleaning solution (e.g., KOH) to limit any potential changes in the surface composition which could arise from the use of cleaning solutions with different chemistries. A near neutral pH (8) cleaning solution was chosen to reach an oil-free/Al-rich state, a medium (10)(11)(12) and a high alkali one (>13) to obtain two partly Al-free states with different contents of aluminum remaining at the surface. The different pH values were selected based on internal data.…”

“…Usually, before application of a pretreatment layer, HDG steel strips are alkaline cleaned. This step is performed to remove any contamination at the surface, such as oils, to improve the wettability of the surface and is also known to dissolve aluminum phases and remove the native oxide film [6,8,13,19,20]. Indeed, the hydroxide ions in the etchant react with the metal-oxide surface and cause the dissolution of the oxide layer and the underlaying metal.…”

Assessment of pitting corrosion in bare and passivated (wet scCO2‐induced patination and chemical passivation) hot‐dip galvanized steel samples with SVET, FTIR, and SEM (EDS)

“…The initial protective properties of HDG are due to the uniformly covering, inert Al 2 O 3 layer on the surface. [ 9 ] However, the corrosion protection achieved by natural simple oxides is short lived. The corrosion processes were visibly continuous after 3‐hr immersion, and with time, the formation rate of pits (black spots detectable with light microscopy) decreased remarkably.…”

“…For zinc coatings, nm‐scale spatial resolution has been achieved. [ 9 ] Argon ion beam milling is capable of producing high‐quality cross‐sections of very thin layers (<100 nm) and abrasion‐sensitive deposits on zinc coatings. [ 10–12 ]…”