Corrosion Behavior Of Zn-ni Coatings Deposited By Electrolytic Plasma Processing

Abstract

In the current research project, three groups of Zn-Ni coatings of various Ni contents were deposited on comercial carbon steel by electrolytic plasma processing (EPP), a novel surface modification technique. The coating characteristics were studied along with their corrosion behavior in order to determine their potential as electroplated Cd coating replacements. The latter electroplated coatings are currently used in aerospace applications but are toxic and hazardous to humans and the environment. Coating surfaces and cross sections were analyzed by scanning electron microscopy (SEM) while compositional analysis was conducted by energy-dispersive spectroscopy (EDS). Phase structure analysis was carried out by x-ray diffraction analysis. Corrosion behavior in tap water and 3.5% NaCl solution was studied by corrosion potential measurements and anodic polarization testing. The results showed that EPP can deposit conformal Zn-Ni coatings with various Ni contents by varying the zinc and nickel salt ratio in the electrolyte via an up normal co-deposition process (i.e., a 50 wt% Ni salt in the electrolyte results in 30 wt% Ni in the coating). The SEM observations showed that the coatings were composed of two phases with different morphologies; a nodular type and a flake type. The XRD analysis showed that the iv nodular Zn-Ni phase was a γ1 Zn-rich phase and the flake type phase a γ2 Ni-rich phase. The γ2 phase was present only in coatings with more than18 wt% Ni and its presence increased with increasing Ni content. Analysis of the electrochemical tests showed an increase in the corrosion potential and a decrease in the corrosion current density with increasing Ni content in the coatings. As one might surmise, corrosion potential was found to be lower and corrosion rate higher in tests conducted in NaCl solution compared to tap water. Post electrochemical testing SEM observations showed that the γ1, Zn-rich phase was most affected by corrosion compared to Nirich, γ2 phase. These findings are in agreement with the XRD and EDS results since the higher Ni content phase is expected to be more noble than the active Zn-rich phase. Comparisons with the electrochemical behavior of 4340 steel showed that only coatings with less than 18 wt% Ni (only γ1 phase present) have a more active potential than that of steels in both tap water and NaCl solution, thus can serve as sacrificial anodes providing protection. Coatings with a higher Ni content (<18 wt% Ni) were found in general to be noble compared to steel (presence of the γ2 phase), thus may accelerate corrosion of the underlying steel if the coating is damaged or cracked. The role of conversion coatings was found to be positive in improving the sacrificial behavior of the low Ni content coatings. Thus, the present study clearly shows that Zn-Ni coatings with <18 wt% Ni deposited by EPP are anodic to steel providing corrosion protection by acting as sacrificial anodes. These coatings possess the potential to replace current electroplated Cd coatings. Finally, application of conversion coatings can provide additional benefits to the Zn-Ni coatings in their corrosion protection role.