How to use cut and etch technique to test a weld
Macro etch test is a technique used to test a weld by revealing the structure of the base material and weld. It is one of the destructive testing methods to evaluate the characteristics of a weld. In this method, the sample is cut to expose the profile of the weld. This will enable the inspection of how the weld is adhering to the base metal and other characteristics like penetration or cracks.
After cutting, the sample is published to reveal create a smooth surface before etching. The cutting process usually leaves a coarse trace on the material and polishing is important to reveal the finer details of the weld.
Profile of a MIG weld after cutting
The polished surface of the weld with a 240 grit sandpaper
After polishing to the required level, various acid solutions can be used to etch the surface and reveal finer details. A combination of ethanol and nitric acid is usually used. Other industrial-grade rust removers can also be used for this purpose (for example, Rust Off from Bunnings Warehouse or other phosphoric acid solutions). Make sure you use proper personal protection equipment (for example gloves and eye protection) and follow the local regulations for disposal of these solutions as they are usually classified as harmful and corrosive materials.
The etchant acid reacting with the metal
The profile after etching revealing the weld boundry
After etching the sample in the acid solution for a few minutes, the boundaries of the base metal and weld become visible. The visual inspection at this point can reveal certain information about the weld. For example, the following photos show the effect of different wire feed rates on MIG welding’s result.
Effect of MIG wire feed speed on the welding tested by cutting and etching
Low magnification microscopes can also be used to examine the finer structure of the weld. For this purpose, the polishing should be done to achieve a finer surface finish before etching.