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Up Topic Welding Industry / Technical Discussions / Plasma cutting and Corrosion
- - By chuckr Date 02-27-2003 17:38
I have been told that when stainless steel is cut with a plasma cutter that the cut edges will rust. Is there any truth to this? To me it makes no sense unless the heat effected zones chemistry somehow changes??

Any help with this would be very much appreciated.

Parent - - By jwright650 (*****) Date 02-27-2003 17:49
Do you know what type of SS they are talking about?
John Wright
Parent - By chuckr Date 02-27-2003 17:50
14 ga. 304

Parent - - By Niekie3 (***) Date 02-27-2003 18:23
Possibly the people are referring to the zone adjacent to the cut that has a "burned" oxide layer. (Same as you get when welding.) As with welding, you should pickle and passivate this zone to restore the full corrosion resistance. This is a surface phenomenon.

I am not aware that the bulk of the material substantially changes its structure, resulting in excess corrosion.

Niekie Jooste
Parent - - By Dschertzer Date 03-03-2003 02:24
What is meant by "pickle and passivate"?
Parent - By TimGary (****) Date 03-03-2003 13:13
Stainless Steel relies on much of it's corrosion resistance on a microscopically thin chromium oxide layer called the passivation layer. This very thin and damageable layer can be destroyed by thermal or mechanical influences such as welding or grinding. Welding produces a chromium depleted oxide layer in the weld zone which has less corrosion resistance than the original material. For this reason, the annealing colors and fire scales must be removed after welding. A good indicator of the formation of a homogeneous and dense passivation layer is a clean and shiny surface. This can be achieved through the pickling process. The further application of a passivation agent builds and intensifies this protection layer for maximum corrosion resistance.
The generalized process is as follows:
1. Clean the surface of all contaminates such as loose scale, oil, grease and surface rust.
2. Apply a pickling agent. This nitric and hydrofluric acid based substance removes all annealing colors, fire scales and corrosion, thus preparing the surface for passivation.
3. Apply the Passivation agent. This is another chemical treatment that restores the protective chromium oxide layer as well as neutralizes pickling acids, thereby stopping the pickling process.
There are many different metohds of performing this process. One company I used to work for used large tanks and vast amounts of chemical solution to treat the entire weldment assemblies at once.
A person working in a small shop or garage may either send the weldment out to a buisness with the required equipment or purchase the chemicals required to do this himself. The later has been simplified into a paste form, that is applied with a brush and washed off with water, by companies such as Intercon and Pelox.
If you decide to do this yourself, please adhere to all of the manufacturer's safety precautions as these are extremely hazordous chemicals and should be treated with the utmost respect.

Parent - By RonG (****) Date 02-27-2003 20:40
Are you useing gas or air? Air will will leave an Oxide surface.
Parent - - By smithdos (*) Date 03-04-2003 17:35
You might be referring to the problem of preferential corrosion of carbide precipitates in the HAZ (in any thermally affected material - welded, cut, etc.).

This is caused (in 300 series "stabilized" stainless) by formation of carbides in the HAZ. If the material goes above the "stabilization temperature" (a function of the alloy type) as it cools back to room temperature various alloying elements react with the carbon and form carbide precipitates. To mitigate formation of carbides, *rapidly* quench the HAZ to within the stabilization temp. Slow cooling is bad in that it causes *more* carbide formation....more carbides = more corrosion.

If you want book references, let me know. It is a well documented problem.

As an aside, some rusting of the material will occur if the part is not passivated.....refer to the other posts regarding that process.


Parent - By Tim Buyle (**) Date 03-05-2003 08:53
Dear Dave,

I believe you are mixing 2 types of corrosion.

(1) "weld decay" : due to forming of chromium carbides at the material grain borders, resulting in less then 12% unbonded chromium so the corrosion properties are destroyed ; this is called intercristaline corrosion (IC) ; corrosion will occur a few millimeters left and right of the weld ; major cause = the level of carbon is too high

You can decrease the level of carbon either by removing it out of the material or by "stabilising" the most of the carbon so it is not harmfull anymore : this is done by addition of titanium or colombium (niobium) ; these elements will form Ti-carbides or Cb-carbides ; Cr-carbides can not be formed anymore (no more free carbon left)

But with "stabilised SS" there's a particular form of IC possible :

(2) "knife line attack" :when heating up the material Ti-carbides and Nb-carbides are dissolved , carbon is free to travel in the material and can form Cr-carbides again ... destroying corrosion properties again ...
, this occurs at the fusion line of the weld and NOT in the HAZ !

Conclusion :

* prefer low carbon stainless steels to weld (indication "L", eg. SS304L)
* if stabilised stainless is used : there's always a risc of knife line attack , but this can be undone by PWHT (heating up and quenching)

All I wanted to say was : corrosion due to carbide forming at stabilised stainless occurs at the fusion line ; corrosion due to carbide forming at high carbon grades occurs in the HAZ ; it seemed you mixed them up ...

I also believe this has nothing to do with corrosion of cut edges.
After cutting there's a very thick chromium oxide layer formed at high temperature, that is full of cracks through wich any medium (water, dust, air) can attack the steel.
Up Topic Welding Industry / Technical Discussions / Plasma cutting and Corrosion

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