Cadmium Substitution - garteur

GARTEUR LIMITED
ANNEX C
Corrosion resistance
C.1 Introduction
Two aspects of the corrosion protection afforded by replacement metal coatings were
investigated,
• performance as a barrier coating
• sacrificial properties of the coating
The range of tests employed and the types of test specimen used are summarised in
table C1.
The accelerated tests and outdoor exposure trials conducted on undamaged panels
were aimed at establishing the overall corrosion resistance of the coatings. The
appearance of red rust was used as one of the criteria in comparing the performance of
the various coatings. This was taken as an indication of the barrier properties of the
coatings, although once the coating has corroded through to the substrate the sacrificial
properties of the coating will be important. Scribed panels were used to semi-quantify the
relative sacrificial properties of the coatings.
Electrochemical measurements enable a more detailed understanding of the barrier and
sacrificial properties of the coatings in saline environments to be established. The
techniques employed allow the corrosion resistance of the coatings to be measured and
the galvanic corrosion current and protection distances to be determined.
C.2 Test procedures
Details of the accelerated test procedures and outdoor exposure trials are given below
together with the electrochemical techniques employed.
C.2.1
C.2.1.1
Accelerated corrosion tests
Exposure to 5% neutral salt fog
Testing was carried out in accordance with ASTM B117 [C1]. Coated test panels were
exposed to a continuous 5% neutral salt spray for up to six weeks in a cabinet
maintained at a temperature of 35 o C. The panels were inspected at regular intervals for
the first signs of red rust.
C.2.1.2
Exposure to intermittent acidified salt spray (MASTMAASIS)
Coated test panels were assessed using an intermittent acidified salt spray test based on
the procedure described by Lifka and Sprowls [C2] and covered by ASTM G85-84 [C3].
The test was originally developed for evaluating the susceptibility of aerospace
aluminium alloys to exfoliation corrosion but has found wider applications in the study of
metal coatings.
In the present work coated panels were exposed for periods of up to 3000hours to
repeated 6 hour test cycles. Each test cycle consisted of a 45 minute spray with 5%
acidified salt solution, a 2 hour direct air purge into the roof of the cabinet during which
the relative humidity fell to between 40 and 45%, followed by a 3 hour 15 minute soak
when the relative humidity rose to 90 to 95%. In tests conducted at DERA the acidity of
GARTEUR SM/AG17 TP128 Page 57