Stainless Steel relies on much of it's corrosion resistance on a microscopically thin chromium oxide layer called the passivation layer. This very thin and damageable layer can be destroyed by thermal or mechanical influences such as welding or grinding. Welding produces a chromium depleted oxide layer in the weld zone which has less corrosion resistance than the original material. For this reason, the annealing colors and fire scales must be removed after welding. A good indicator of the formation of a homogeneous and dense passivation layer is a clean and shiny surface. This can be achieved through the pickling process. The further application of a passivation agent builds and intensifies this protection layer for maximum corrosion resistance.
The generalized process is as follows:
1. Clean the surface of all contaminates such as loose scale, oil, grease and surface rust.
2. Apply a pickling agent. This nitric and hydrofluric acid based substance removes all annealing colors, fire scales and corrosion, thus preparing the surface for passivation.
3. Apply the Passivation agent. This is another chemical treatment that restores the protective chromium oxide layer as well as neutralizes pickling acids, thereby stopping the pickling process.
There are many different metohds of performing this process. One company I used to work for used large tanks and vast amounts of chemical solution to treat the entire weldment assemblies at once.
A person working in a small shop or garage may either send the weldment out to a buisness with the required equipment or purchase the chemicals required to do this himself. The later has been simplified into a paste form, that is applied with a brush and washed off with water, by companies such as Intercon and Pelox.
If you decide to do this yourself, please adhere to all of the manufacturer's safety precautions as these are extremely hazordous chemicals and should be treated with the utmost respect.

TIm