Cadmium Substitution - garteur

GARTEUR LIMITED
ANNEX F
Stress corrosion cracking studies
F.1 Introduction
High strength steels are susceptible to hydrogen embrittlement and stress corrosion
cracking. Conventional aqueous electrodeposition processes are less than 100%
efficient and during electroplating some hydrogen may diffuse into the steel component.
This may lead to failure of the component under tensile loads as a result of hydrogen
embrittlement. In order to minimise the risk of failure, components are baked after
plating, typically at temperatures of between 190 and 230 o C. The duration of the deembrittlement
treatmen is dependent on the ultimate tensile strength of the steel. For
steels with maximum tensile strengths in the range 1101 MPa to 1450 MPa the
European Standard EN 2133 advises a minimum baking time of 8 hours after cadmium
plating [F1]. Steels in the strength range 1451 to 1800 MPa require longer baking
treatments usually at least 18 hours whilst for ultra high strength steels, i.e. above 1801
MPa, times in excess of 24 hours must be employed [F2].
Under certain conditions, parts manufactured from high strength steels may fail by stress
corrosion cracking. For failure to occur the component must be under tensile stress and
be exposed to a corrosive environment. The mechanism of stress corrosion cracking in
high strength steels is normally hydrogen embrittlement. Hydrogen may be introduced
into steels as a result of corrosion processes occurring on the surface of the component.
A damaged coating, for example, may lead to the establishment of a galvanic corrosion
cell between the coating and steel substrate. Hydrogen generated as a result of the
cathodic reaction may diffuse into the steel, ultimately leading to hydrogen embrittlement
failure.
In the current programme both aspects of hydrogen embrittlement have been
investigated.
F.2 Test procedures
F.2.1
Hydrogen embrittlement
Sustained load testing and slow bend tests were carried on a limited number of coated
and heat treated samples to establish whether the recommended de-embrittlement
treatments were effective [F3].
Notched tensile specimens were manufactured from AISI 4340 steel heat treated to 1790
- 1930 MPa. The specimens were then electroplated with zinc-nickel and subsequently
de-embrittled by heat treating at 190 o C for 3 hours. Two series of 3 test pieces were
placed under sustained load for 200 hours at 75% of the notched tensile strength. The
tests were conducted at room temperature.
A limited number of slow bend tests were conducted by Aerospatiale on a 35NCD16
steel heat treated to 1800MPa. The tests were conducted in accordance with prEN 2831.
The initial failure angle, α 0 , and the failure angle after surface treatment, α 1 , were
measured. Details are given in reference [F4].
F.2.2
Stress corrosion cracking
The risk of stress corrosion cracking occurring as a result of corrosion has been studied
using notched tensile specimens. These were loaded to 75% of the notched tensile
strength after coating, and are then subjected to alternate immersion in 3.5% sodium
chloride solution until failure occurs. The performance of each of the coatings has been
GARTEUR SM/AG17 TP128 Page 85