Cadmium Substitution - garteur

GARTEUR LIMITED
C.3.2.3
Electrochemical measurements
Results of the OCP measurements are presented in table C11. In general the zinc based
coatings (ED zinc-nickel, ED zinc-cobalt-iron and Delta-tone) exhibit much lower
potentials than the aluminium coatings. On this basis it would be expected that the zinc
rich coatings would provide a greater level of sacrificial protection and this is supported
by the accelerated corrosion tests and outdoor exposure trials on scribed panels.
The average galvanic corrosion currents measured using the Scratch Model specimens
are summarised in table C12. In broad agreement with the findings of the OCP studies
the as deposited zinc based coatings show higher levels of galvanic corrosion current
than the pure aluminium coatings and SermeTel 984. Measurements made on the UBMS
aluminium - magnesium coating confirm that this is a more sacrificial coating than pure
aluminium.
Data obtained from the cathodic protection measurement technique were used to
calculate the protection distance for each of the coatings studied. This represents the
distance over which a coating can provide sacrificial protection to the steel strip. The
results are presented in table C13. This shows that the two pure aluminium coatings are
relatively poor sacrificial coatings whilst the zinc based coatings provide protection over
much greater distances than cadmium plating. The aluminium - magnesium coating
shows a significant improvement over pure aluminium and is more effective than
cadmium plating.
C.4 Conclusions
1. Overall the aluminium based coatings were found to have superior barrier properties
to the zinc rich deposits studied in this investigation.
2. Electrochemical and accelerated corrosion tests have demonstrated that the zinc rich
deposits are more effective in preventing the onset of rusting at scratches or defects
in the coating than pure aluminium or aluminium alloy coatings.
C.5 References
[C1]
[C2]
[C3]
Standard method of salt spray (fog) testing, ASTM B117-73 published by the
American Society for Testing and Materials, Philadelphia, Pa. 19103 (1973)
Lifka B.W. and Sprowls D. "An improved exfoliation test for aluminium alloys",
Corrosion 22 p7 (1966)
Modified salt spray (fog) testing, ASTM G85-84 published by the American
Society for Testing and Materials, Philadelphia, Pa. 19103 (1984)
[C4] Standard method for Testing Coated Metal Specimens at 100% Relative
Humidity, ASTM D2247-68 published by the American Society for Testing and
Materials, Philadelphia, Pa. 19103 (1980)
[C5]
[C6]
Smith C.J.E., Baldwin K.R., Hewins M.A.H. and Gibson M.C., “A Study into the
Corrosion Inhibition of an Aluminium Alloy by Cerium Salts” published in
“Progress in the Understanding and Prevention of Corrosion” pp1652 - 1663
(1993)
Jansen E.F.M. and 'tHart W.G.J., Corrosion properties of cadmium, ZnNi, ZnCoFe
and SERMETEL coatings - Garteur Cadmium Substitution programme NLR
Progress Report 1- NLR CR 95607 L (1995)
GARTEUR SM/AG17 TP128 Page 61