Does the electric current during spot weld pay an important role to the rust ?

On which side does the rust occur? What kind of metals?

Just theorizing: Maybe due to the high current, galvanic corrosion occurs faster... Or due to the higher current --> higher heat --> protective coating of the metal breaks down?

As a pointer, heat-affected-zones [HAZ] are set up in steels of various kinds when welding takes place, with the crystalline structure changed in those areas when compared to the unwelded metal. Under some circumstances the HAZ can become more prone to chemical attack. A discussion with a metallurgist colleague might yield further information?

Hi,

The comment regarding the HAZ is correct. The HAZ area adjacent to the weld is proportional to the heat applied. The higher the current the greater the heat generated, the weld time is decreased with the increase in current the greater the HAZ.

The corrosion is due to the formation of carbides.

Using of excessive current by about 33% causes a burning at HAZ. A complete UT and hardness measuring at the burned areas is required.

If your friend is trying to spot weld a hex head bolt to sheet metal surface by the top of its head to create a stud for attaching other items, tell him to STOP!

He should use welding studs rather than headed bolts. These studs are headless bolts that have instead a small round "head" with a nipple projecting from the top. They are used by inserting the threaded end into an electric "stud gun". When you pull the trigger on the stud gun while pushing straight against the sheet metal the small nipple becomes the hot spot and melts. This metal melt flows between the stud and the sheet metal making a very thin, but very strong weld, with relatively low current. They may be slightly pricier than standard hex head bolts, but the savings in reduced scrap from the rust problem or from bad welds should offset the price. Also, you do not need a trained welder to operate the "stud gun". It is so easy, a cave man could do it!

If he must use hex head bolts for some reason, perhaps a lower temperature brazing may be a solution rather than welding.

Another possibility, if the rusting is from immediate oxidation due to the higher temperature and not later galvanic or other corrosion, could be to use a nitrogen "blanket", i.e flooding the area near the weld with nitrogen gas immediately prior to welding, to exclude oxygen until the area has cooled.

Dear All thank you very much for your concerned.

Regarding Welding Bolt. I believe they are using Round shape with small pin . During spot welding, Metal pin on the bolt will be melted and weld the bolt head and sheet together. I hope u guys can image it. ( sorry I am not good in describe the process in detail)

As from your comments, I believe high possibility should come from over excess current that caused HAX to burned up as well as create immidiate oxidation. (Rust occur in just 2 hours after process).

The materials they are using are JSD270D JSC270F. Rust occurred on sheet only.