While tribocorrosion phenomena may affect many materials, they are most critical for metals, especially the normally corrosion resistant so-called passive metals. The vast majority of corrosion resistant metals and alloys used in engineering (stainless steels, titanium, aluminum etc.) fall into this category. These metals are thermodynamically unstable in the presence of oxygen or water and they derive their corrosion resistance from the presence at the surface of a thin oxide film, called the passive film, which acts as a protective barrier between the metal and its environment. Passive films are usually just a few atomic layers thick. Nevertheless, they can provide excellent corrosion protection because if damaged accidentally they spontaneously self-heal by metal oxidation. However, when a metal surface is subjected to severe rubbing or to a stream of impacting particles the passive film damage becomes continuous and extensive. The self-healing process may no longer be effective and in addition, it requires a high rate of metal oxidation...
   Please excuse the plagiarism, but the study was able to impart the knowledge much better than I could. Try cutting out the CO2 and go to 100% argon. Someone told me once that welding was technique, electronics, and chemistry. If something is going wrong it will probably be caused by one of them.