Electrodeposition of corrosion resistant zinc alloy coatings
With the increase use of de-icing salts on roads for safety, the need for improved
corrosion resistance of the traditional galvanized automobile bodies has never been
greater. In the present work, Zn alloy coatings (Zn-Ni and Zn-Co) were studied as an
alternative to pure Zn coatings.
The production of these deposits involved formulation of various acidic (pH of about
5.5) chloride based solutions. These showed anomalous deposition, that is, alloys were
deposited much more easily than expected from the noble behaviour of Ni and Co metals.
Coating compositions ranging from 0 to about 37% Ni and 20% Co were obtained. The
chemical composition of the coatings depended very much on the electrolytes nature and
operating conditions. The Ni content of deposits increased with increase in Ni bath
concentration, temperature, pH and solution agitation but decreased considerably with
increase in current density. The throwing power of the Zn-Ni solution deteriorated as Ni
metal bath concentration increased.
The Co content of deposits also increased with increase in Co bath concentration and
temperature, and decreased with increase in current density. However, the addition of
commercial organic additives to Zn-Co plating solutions suppressed considerably the
amount of Co in the coatings. The Co content of deposits plated from Zincrolyte solution
was found to be more sensitive to variation in current density than in the case of
deposits plated from the alkaline Canning solution.
The chromating procedures were carried out using laboratory formulated solution and
commercially available ones. The deposit surface state was of great significance in
influencing the formulation of conversion coatings. Bright and smooth deposits acquired
an iridescent colour when treated with the laboratory formulated solution. However, the
dull deposits acquired a brownish appearance.
The correlation between the electrochemical test results and the neutral salt spray in
marine environment was good. Non-chromated Zn-Ni coatings containing about 11-14%
Ni increased in corrosion resistance compared to pure Zn. Non-chromated Zn-Co
deposits of composition 4-8% were required to show a significant improvement in
Corrosion resistance was improved considerably by conversion coating. However, the
type of conversion coating was very important. Samples treated in a laboratory solution
performed badly compared to those treated in commercial solutions. Zn alloy coatings
were superior to pure Zn, the Schloetter sample (13.8% Ni) had the lowest corrosion
rate, followed by the Canning sample (1.0% Co) and then Zincrolyte (0.3% Co).
Neither the chromium content of the conversion films nor the chromium state was found
to have an effect on corrosion performance of the coatings.