Why Do Welded Joints Rust Faster?

[sci-phy]

Hey ,
Earlier today, I discovered that on a metal frame in my house, rust was accumulating on the weld whereas other parts seemed ok. Some ppl told me that it was because the welded region was hotter and hence, it rusts faster. But I don't get it... The welded joint has been cool and at the same temperature as the rest of the frame for years. Then why do they rust faster??

 

[jambaugh]

I'm not sure of the correct answer but here are some possible effects.

Firstly the welding material may be slightly different alloy from the base which can set up net galvanic action. Like a weak battery being short circuited. It rusts as it "discharges". You see this strongly where you couple brass or copper to iron water pipes.

Secondly if the frame metal had a surface treatment to inhibit corrosion the heat of welding would burn it off.

Thirdly (and my suspicion) is that, steel being a carbon iron alloy, the heat of welding ruins the temper causing microscopic clumping of carbon molecules. These on the surface create points where rust starts. They form little galvanic sites where you get pitting rust instead of a more protective patina of oxide.

 

[streeters]

Hey ,
Earlier today, I discovered that on a metal frame in my house, rust was accumulating on the weld whereas other parts seemed ok. Some ppl told me that it was because the welded region was hotter and hence, it rusts faster. But I don't get it... The welded joint has been cool and at the same temperature as the rest of the frame for years. Then why do they rust faster??

I'm assuming its a stainless steel frame, which will have an appreciable amount of chromium in it (13-18%). This makes it corrosion resistant by forming a stable chromium oxide layer on the surface. There needs to be a minimum amount of chromium in solid solution for the metal to 'heal' itself if the oxide layer is damaged.

Welding adds heat, obviously, and allows chromium carbides and carbonates to form amongst other things. These carbides use up a lot of the chromium in solid solution so you have regions of chromium depletion around the welds, allowing corrosion.

 

[chemisttree]

I'm not sure of the correct answer but here are some possible effects.

Firstly the welding material may be slightly different alloy from the base which can set up net galvanic action. Like a weak battery being short circuited. It rusts as it "discharges". You see this strongly where you couple brass or copper to iron water pipes.

Secondly if the frame metal had a surface treatment to inhibit corrosion the heat of welding would burn it off.

Thirdly (and my suspicion) is that, steel being a carbon iron alloy, the heat of welding ruins the temper causing microscopic clumping of carbon molecules. These on the surface create points where rust starts. They form little galvanic sites where you get pitting rust instead of a more protective patina of oxide.

I agree with all of this but in addition, the weld areas are under significant stress from the welding operation and stress causes faster corrosion.