Corrosion resistance of the transition layer and corrosion-resistant layer in composite plate weld joints

Then the corrosion resistance of the corrosion-resistant layer must be good, right? Otherwise, what’s the point of having such a layer?

The transition layer merely serves to dilute; the resistance to etching is still provided by the corrosion-resistant layer

The transition layer primarily focuses on the bonding between the base layer and the corrosion-resistant layer, without considering corrosion resistance

But in terms of chemical composition, the alloy content in the transition layer is higher; therefore, its corrosion resistance should be better, right? Put another way, if the transition layer weld is too thick while the corrosion-resistant layer is very thin or absent, does this have a significant impact on the equipment’s lifespan?

Corrosion resistance does not depend on the amount of alloy present; rather, it is relative to the medium in question； When there are many alloying elements, the weld structure is bainite or low-carbon martensite; whereas the weld structure of austenitic stainless steel consists of austenite with a small amount of ferrite. Therefore, their corrosion resistance differs.

The alloy content in the transition layer is higher than that in the corrosion-resistant layer, as it needs to be diluted. The dilution effect in surfacing is somewhat similar to diffusion, as elements move from areas with high concentrations to those with lower concentrations; in other words, the alloying elements diffuse from the transition layer to the base layer, thereby making the alloy content in the transition layer similar to that in the corrosion-resistant layer. This ensures that the corrosion-resistant layer retains its appropriate alloying elements. If the transition layer is too thin, it will dilute the alloying elements in the corrosion-resistant layer, preventing it from meeting the corrosion resistance requirements. If the transition layer is made too thick, its economic cost will increase, as transition layer electrodes are more expensive than corrosion-resistant layers. Furthermore, if the thickness of the transition layer weld is too great while the thickness of the corrosion-resistant layer is very thin or even absent, whether this affects the lifespan of the equipment should be determined based on weld evaluation. Based on the weld evaluation tests we have conducted, the corrosion resistance of the transition layer is inadequate, and its thickness is also unsatisfactory; the exact reasons are not clear.

The function of the surface layer is to resist corrosion by media, while the function of the transition layer is to prevent dilution.

This cannot be compared, as the transfer of C atoms has to be taken into account; therefore, in general, the amount of alloy in the transition layer is greater than that of the elements in the clad layer. The reason why transition-layer welding electrodes are not used exclusively is due to cost considerations!