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
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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.