Is it true that the welding of stainless galvanized steel can cause weakening?
When extreme levels of protection against corrosion are required, stainless steel is sometimes galvanized. This provides great corrosion protection; However, the welding of such corrosive protection systems is a concern. When certain types of metals become molten, they may cause cracks in other types of metals. Some examples of this include mercury, which causes cracking in copper alloys, cadmium which causes cracking in titanium, zinc and aluminum, lead which causes cracking in austenitic stainless steels. (R.W. Staehle, Stress Corrosion Cracking and Hydrogen Embrittlement of Iron BaseAlloys. Houston, TX: NACE, 1977.)
Since the galvanized coating is applied for corrosion protection, it should never become molten, but there are two cases where this can happen. The first example is when galvanized stainless steel is welded. This might also happen when stainless steel is welded to galvanized steel, or vice versa. The second case is that of galvanized steels zinc coating coming into contact with stainless steel at temperatures that make zinc melt. This is not an environment suitable for galvanized steel and will not be discussed here. Let’s discuss the first case in more detail. When galvanized stainless steel is welded or when galvanized steel and stainless steel are welded together, the zinc becomes molten and can penetrate intergranular stainless steel, thereby causing cracking of the liquid metal (LMC). An example of this is discussed on page 179 in the NACE book Fundamentals of Designing for Corrosion Control: "A Corrosion Aid for the Designer."�
To remedy this situation, remove zinc from the area to be welded prior to welding. The document "Funndamentals of Designing for Corrosion Control" : "A Corrosion Aid for the Designer" explains that the best method to remove the zinc is applying 20% hydrochloric acid (or other acid) and then rinsing-drying the area before welding. The most common method for grinding zinc off of the area to be welded may leave zinc particles on the area to be welded, which can then result in LMC. Hot-dip galvanizing is not the only type of zinc coating that can cause LMC on stainless steel. All other types of zinc coatings, including zinc-rich paint, can cause LMC when heated to melting point and coming into contact with stainless steel.
Usually, the galvanizer is several steps removed from the planned erection or application process, but talking to your client can prevent a costly or dangerous situation. If you galvanize stainless steel for your client, it is important to advise of potential LMC if the steel is welded later.
Source: AGA - American Galvanizers AssociationReturn to news