Testing Welds - Did it Penetrate? The macro etch test is a very valuable test to verify or check for penetration. It is especially valuable when developing a procedure or welding technique. Since it only tests one cross section, it is not always conclusive unless multiple cuts are done for a good sampling. Carbon Steel and Aluminum are relatively easy to etch but other more corrosion resistant alloys like stainless, nickel, titanium, and cobalt alloys require specific etchants and sometimes requ