Unlike with mechanical joints, there are no corrosion prone crevices. Also no stress corrosion has been reported to date. Still a question often raised is how Triclad behaves in a marine environment relative to galvanic corrosion, given two metals with considerable differences in potential. It is clear that a suitable paint or coating will prevent contact between the electrolyte and the strip, thus avoiding corrosion. It is however worthwhile investigating more closely the effects of the absence of such protection.
Considering the lower galvanic potential of the steel, extreme corrosion of the aluminium may be anticipated; particularly near the interface. This is the area where the metal has been heavily worked and the anode is in close proximity. Initial corrosion tests on unpainted samples of approximately equal aluminium to steel areas revealed however a natural insulating effect.As expected, slight penetration began at the interface as the aluminium started to corrode. But, instead of acting as a latent area of high ion concentration and thereby accelerating corrosion, the penetration area gradually filled with an extremely hard and inert corrosion product, aluminium oxide hydrate. The oxide acted as a seal and rendered the system passive after only a very minor penetration; the exact level dependent upon the severity of the initial corrosive environment.Accelerated salt-spray tests, simulating years of exposure, further demonstrated that corrosion became negligible after the initial barrier had been built up. Painted samples, whose interface had been scratched so as to expose only a small area, were subjected to the same testing environments. With these, the only interface corrosion was a slight pinpoint area beneath the scratch. The solid metallurgical bond restricted the electrolyte from penetrating the interface, while the build up of corrosion product prevented extensive pitting. This served to prove the transition joint system’s advantage over mechanical connections. In the latter, a crevice exists between the faying surfaces and once the protective coating is broken, the electrolyte rapidly penetrates the interface.
Aluminium steel welding | What is a structural transition joint? | What is explosive bonding? | What is explosion welding? | What is explosive cladding? | The application of Triclad in the marine industry | Welding parameters: general guidelines | Fabrication guidlines | Corrosion resistance | What is Triclad? | General background of explosion cladding | Joining aluminium to steel | Welding aluminium to steel: Triclad