Why 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.

 

[LudusRex]

I can explain the reason behind welded joints rusting faster. When two pieces of metal are welded together, the process involves heating the metals to a very high temperature and then joining them together. This high temperature causes the metal to undergo a process called annealing, which changes the chemical and physical properties of the metal in the welded region.

One of the changes that occur during annealing is the formation of a new grain structure in the metal. This new grain structure is different from the original structure of the metal and is often weaker and more susceptible to corrosion. Additionally, the high temperature during welding can also cause the formation of small cracks and imperfections in the welded joint, providing a pathway for moisture and oxygen to penetrate and initiate the corrosion process.

Furthermore, the welding process can also introduce impurities and contaminants into the welded joint, which can accelerate the corrosion process. These impurities can come from the welding materials, such as filler metals, or from the surrounding environment.

In contrast, the rest of the metal frame that has not been welded does not undergo the same changes in grain structure and does not have the same level of imperfections, making it less prone to corrosion.

In summary, the combination of changes in grain structure, imperfections, and introduction of impurities during the welding process makes welded joints more vulnerable to corrosion compared to the rest of the metal frame. To prevent or slow down the rusting process, proper surface preparation and coating techniques can be used during the welding process to protect the welded joint from corrosion.