Cadmium Substitution - garteur

GARTEUR LIMITED
ANNEX D
Galvanic compatibility
D.1 Introduction
An important feature of cadmium plating is its capacity to minimise the risk of galvanic
corrosion occurring between steel fasteners and aerospace aluminium alloys. The
electrochemical potential of cadmium plating is close to that for aluminium, so that the
driving force for galvanic corrosion is small. Any cadmium replacements intended for use
on fasteners must similarly reduce the risk of galvanic corrosion.
In the present study two approaches have been employed to establish the galvanic
compatibility of cadmium substitute coatings with aerospace aluminium alloys. The first
approach has been to expose combinations of coated fasteners inserted into 2000 and
7000 series aluminium alloy test blocks and coupons, to both accelerated corrosion tests
and to natural exposure conditions. The second approach has been to determine the
galvanic currents generated between different coatings and aluminium alloys using
electrochemical measurements.
D.2 Test procedures
D.2.1
Accelerated corrosion tests
Test specimens comprising of coated fasteners inserted into aluminium alloy blocks were
exposed to continuous neutral salt fog. Two different designs of specimen were
employed by Daimler Benz Airbus Aerospace and Fokker. Details of the arrangement
used by Fokker are given in figure D1. After testing the specimens were disassembled
and evidence of coating degradation and corrosion attack of the aluminium blocks was
recorded.
D.2.2
Outdoor exposure trials
Tests were carried out by Fokker at the Schiphol exposure site in The Netherlands. The
specimen configuration was the one used in the neutral salt fog tests and illustrated in
figure D2. The test blocks were removed after 10 months and examined for evidence of
corrosion.
D.2.3
Galvanic corrosion current measurements
The galvanic compatibility of the coatings with two aerospace aluminium alloys was
investigated. The aluminium alloys selected for this study were an aluminium - copper
alloy 2014-T6 and an aluminium - zinc - magnesium alloy, 7075-T6, both of which are
used extensively in aircraft construction. Precise details of the galvanic coupling
experiments have been given elsewhere [D1]. Figure D3 shows schematically the
experimental arrangement employed. Briefly the test electrodes were held parallel and
facing each other in a Perspex holder so that the gap between them was fixed at 25mm.
They were then immersed in quiescent 600mmol l -1 sodium chloride solution (250ml) at
25 o C. A zero-resistance ammeter was used to measure the current flow between the two
electrodes. The experiments were run for 7 days.
After the 7 day period, the aluminium alloy electrodes were removed from the sodium
chloride solution and rinsed in distilled water. The aluminium alloy electrodes were then
chemically cleaned in a hot chromic/phosphoric acid bath, rinsed in distilled water and
acetone, and air-dried. After chemical cleaning, the weight change due to corrosion was
calculated by subtracting the weight determined before coupling from the weight of the
aluminium alloy after coupling and chemical cleaning.
GARTEUR SM/AG17 TP128 Page 71