Cadmium Substitution - garteur

GARTEUR LIMITED
was then immersed in 600 mmol l -1 sodium chloride solution at 25 o C for a period of two
weeks, during which time the galvanic current was monitored using a zero resistance
ammeter and chart recorder.
C.2.3.4
Cathodic protection measurement technique
C.3 Results
The technique involves the measurement of potentials along a steel strip immersed in 6
mmol l -1 sodium chloride solution at 25+ 2 o C. The coating of interest is attached at one
end of the strip as shown in figure C2. The potential along the strip is measured via a
series of 11 saturated calomel electrodes each independently connected to a data
logger. The SCEs were at the following distances from the edge of the coating;
0, 5, 10, 17, 22, 28, 40, 60, 90, 120, 150mm
Potential measurements along the steel strips were carried out for up to 1500 hours.
Further details of the technique are given at reference C10.
C.3.1
C.3.1.1
Barrier properties
Accelerated corrosion tests
a) Resistance to neutral salt fog
Neutral salt spray tests were conducted by three laboratories DERA, Shorts and Fokker
and the results in terms of the time to red rust are given in Table C5.
Although the actual times to red rust for undamaged panels varied considerably between
the three laboratories, there was general agreement regarding the relative performances
of the zinc based coatings and the SermeTel coating. The data show that only the
metallic ceramic coatings, SermeTel out performed cadmium when tested in the as
plated condition. In every case carrying out post plating treatments such as passivating
considerably improved the performance of the coating but only the SermeTel 984 coating
with SermeTel 985 gave a higher time to first red rust than passivated cadmium plating.
As plated
SermeTel 984 > ED cadmium >ED zinc – nickel ~ ED zinc – cobalt – iron
Plated + post plating treatment
SermeTel 984 + 985 > Pass. ED cadmium > Pass. ED Zn-Ni ~ Pass. ED Zn-Co-Fe
Tests on aluminium and aluminium based coatings and Magnicoat were only carried out
at DERA. From Table C5 it is apparent that whilst the as plated UMS Al-Mg, Magnicoat
and SermeTel coatings out perform cadmium plating in the neutral salt fog tests, the
passivated cadmium plating gives the highest level of corrosion protection.
b) Exposure to intermittent acidified salt spray (MASTMAASIS)
MASTMAASIS testing was undertaken by DERA and NLR and the results are
summarised in table C6. Whilst good agreement was obtained for tests conducted on as
plated panels, some differences were found on tests carried out on passivated panels.
GARTEUR SM/AG17 TP128 Page 59