Why stainless steel can not be welded with carbon steel welding rod is because doing so will cause stainless steel to produce intergranular corrosion.

What is intergranular corrosion?
Intergranular corrosion is a kind of local corrosion, which is spread inward along the boundary between metal grains. Due to the difference in the chemical composition between the grain surface and the interior and the existence of grain boundary impurities or internal stress, it will destroy the bond between the grains, greatly reduce the mechanical strength of the metal, the deterioration of mechanical properties, and can not withstand percussion, which is a very dangerous corrosion.

Carbon is the main element that causes intergranular corrosion, usually the carbon content of carbon steel is higher than stainless steel, which is the reason why stainless steel welding with carbon steel electrode is easy to cause intergranular corrosion, so the welding of stainless steel, should choose low-carbon or even ultra-low carbon stainless steel electrode, such as O 102, O 132, O 002 and so on.
In addition, for important structures with high requirements for impact toughness and crack resistance, acid electrode is not suitable. The so-called acid electrode refers to the electrode that contains a large amount of acid oxides (such as silicon dioxide, titanium dioxide, etc.) in the coating.

This is because: (1) the oxide in the acid electrode makes it more oxidizing, and the weld metal contains more oxygen, which makes the alloying element burn larger; (2) Acid electrode coating contains more silica, and some silicon exists in the weld in the form of silicon oxide inclusions, resulting in poor mechanical properties of the weld, especially lower plasticity and impact toughness than alkaline electrode; ③ The acid electrode coating does not contain fluorite (CaF2), and the dehydrogenation ability is low; The weld metal has high hydrogen content and poor cold cracking resistance, and the desulphurization effect is poor because the desulphurization element manganese content in the weld is low.

In summary, acidic electrode should not be used for welding important structures, but alkaline electrode with low oxygen content of weld metal, large alloy transition coefficient, low sulfur content and low hydrogen content should be used.