Surface Oxidation of the High-Strength Steels Electrodeposited with Cu or Fe and the Resultant Defect Formation in Their Coating during the Following Galvanizing and Galvannealing Processes

Abstract

This study examined the surface oxidation of high-strength steels electrodeposited with Cu or Fe and the resultant defect formation in their coating during the following galvanizing and galvannealing processes. The high-strength steels were coated with an Cu or Fe layer by the electroplating method. Then, the coated steels were annealed in a reducing atmosphere, dipped in a molten zinc, and finally transformed into galvannealed steels through the galvannealing process. The formation of Si and Mn oxides on the surface of the high-strength steel was effectively suppressed, and the density of surface defects on the galvanized steel was significantly reduced by the pre-electrodeposition of Cu and Fe. This effect was more prominent for the steels electrodeposited at higher cathodic current densities. The finer electrodeposit layer formed at higher cathodic current density on the steels enabled the suppression of partial surface oxidation by Mn or Si and better wetting of Zn on the surface of the steels in the following galvanizing process. Furthermore, the pre-electrodeposited steels exhibited a smoother surface without surface cracks after the galvannealing process compared with the untreated steel. The diffusion of Fe and Zn in the Zn coating layer in the pre-electrodeposited steels appears to occur more uniformly during the galvannealing process due to the low density of surface defects induced by oxides.

Improvement in Coating Quality and Corrosion Characteristics of Galvanized Steels by Pre-Electrodeposition of Cu

“…The slow kinetics of formation of amorphous-Si-containing oxides under a particular reduction annealing atmosphere, which can be attributed to electrodeposition, has been previously proposed to explain this beneficial effect. 29 The strong reduction in the concentration of coating defects with the applied electrodeposition current intensity supports this hypothesis. It is known that the nucleation and growth rate of Cu particles during the electrodeposition are mainly influenced by the applied cathodic overvoltage.…”

“…This current density was found to be optimum to effectively decrease the degree of selective surface oxidation during the reduction annealing while reducing the coating defects after the galvanization process. 29 As a result, the coating mass of the galvanized steel was increased by about 19% (from 40.9 to 48.6 g m −2 , Table I). As shown in Figs.…”

“…These cracks and pin-holes resulted from Si-and Mn-containing oxides, respectively, during reduction annealing prior to galvanization. 29 Minor alloying elements in dualphase steels oxidize because of their lower standard Gibbs free energy change ( G • ) as compared to that of Fe oxidation reaction under reduction annealing conditions. These oxide compounds, especially Remarkably, these unwanted coating defects were significantly reduced (more than tenfold) by pre-electrodepositing Cu prior to reduction annealing, as shown in Table I.…”

“…The samples were then masked with polypropylene tape, leaving an exposed area of 55 cm 2 . 29 Cu was galvanostatically electrodeposited over the steel sheets for only 1 s at 120 mA cm −2 in a bath containing 0.8 M CuSO 4 .5H 2 O + 0.4 M H 2 SO 4 at 25 • C. The corresponding electrodeposition potential was −0.1 V SCE and the amount of Cu electrodeposited was approximately 0.2-0.4 g m −2 . Following electrodeposition, reduction annealing was carried out at 800 • C for approximately 1 min in a reducing atmosphere of 15% H 2 in N 2 (v/v) with a dew point of −30 • C. The steel sheets were then dipped into a molten Zn pot containing a maximum of 0.3 mass% Al.…”

“…[25][26][27][28] We recently demonstrated that formation of Mn and Si oxides on the surface of a dual-phase steel can be effectively suppressed during reduction annealing by pre-electrodepositing Cu or Fe. 29,30 The amount of electrodeposits on the steel sheets ranged from 0.2 to 0.4 g m −2 , corresponding to a thickness of 22-50 nm, was enough to significantly reduce the surface defects on the final galvanized coatings.…”