Gentlemen and ladies,
We are fabricating some structural fabs out of 304 stainless steel. We are stick welding these fabrications with 308L welding rods. In order to handle any contamination (we are originally a carbon steel fab shop) and to rid ourselves of the oxidized layer around the weld areas we are planning on blasting these fabrications either with glass bead, pecan shell, garnet or sugar sand. My question is are we handling this in the right way, is there another way to better handle this. I have gotten quotes on passivating the surfaces but this is far more expensive than the blasting option. You opinions would be greatly appreciated.
D. Sanders
Pickling / Passivating the surface is the ONLY method to restore the full corrosion properties of stainless steel. Other methods (mechanical) : blasting, grinding, brushing, ... will not remove the oxide layer completely although the surface looks blank. For critical corrosion apllications (storage tanks, vessels, containers,...) the chemical method is the only option. For general construction blasting can be a sollution.
Could you explain the passivating process? Does this use an acid to remove the oxide layer? How does removing the oxide layer restore corrosion properties? What type of acid do you use?
Thanks
kam
PICKLING of stainless steel can be done as follows :
* pickling fluid : (22 weight % HNO3 + 5 weight % HF) at 20° Celcius
* pickling time = 1 to 4 hours for conventional austenitic stainless steel
* every 10° Celcius decrease in temperature doubles the pickling time (as a rule of thumb)
Pickling will remove the relatively thick and porous oxide layer after welding. It is of great importance to clean thoroughly with demineralised water (watch out with Cl in the water !!!) to remove the pickling fluid.
PASSIVATING of stainless steel can be done as folows :
(1) chemical restored oxide layer
(2) naturally restored oxide layer by exposure to fresh air
The chemical method can be done with (19 weight % HNO3 + 2 weight % hydrogen peroxide with citric acid) But I read in some technical papers that this method is NOT superior to passivating in fresh air !!!
One thing to remember : try to avoid the formation of uncontrolled oxide layers due to welding by means of appropriate protection (backing gas, shielding gas, minimise heat input) This will result in less pickling ...
Tim,
Why is mechanical cleaning not sufficient?
Is it not sufficient to say that if we don't see any change in color(meaning that a change in color represents high temperature oxidation), we assume we have restored the material to a "Pristine" condition?
Isn't the oxidation of chromium,which gives rise to the corrosion resistance properties of stainless, a natural phenomenon?
I am of the understanding that the thickness of the oxide layer is due to the "transport of metal ions through the oxide layer to the surface" ( taken from PHYSICAL METALLURGY BY PETER HAASEN).
Do all film protected materials maintain their respective protective coatings in this manner? How about Copper-Nickel?
What differentiates high tempertature oxidation from normal oxidation?
Are the resultant byproducts of high temperature oxidation insulative in nature?
I ask because you seem so adamant about the restoration of a "film" protected material. And I need to know.
Dale Simonds
I'm not trying to put on the spot; to the contrary, I'm hoping you have some information I can use.
Dear Dale,
Stainless steel can withstand (oxidising) corrosion environments due to the fact that it contains a minimum of 12% chromium that is freely distributed in the material and not bonded to any other element. For most stainless steels the chromium content varies between 12 and 30 %.
At the surface the free chromium atoms will bond with oxygen from the air, resulting in a very thin en very tight chromium-oxide layer. This is the protective layer.(atmospheric oxidation)
Some things can go wrong :
(1) when heating up the stainless steel in an oxidising atmoshpere or during cooling down after welding the oxide layer becomes thicker and tougher(high temperature oxidation). It is not tight anymore and shows some micro cracks. The oxide layer is more sensitive to corrosion now : it has to be removed !!! And a new oxide layer must be formed. Both mechanical methods as chemical methods can remove thick oxide layers.
(2) carbon is a tricky element for stainless steel. One carbon atom can bond a lot of chromium atoms. Carbon pick up or carbon migration in the stainless steel will result in "consuming" chromium : chromium is bonded now and not freely distributed any more. The free chromium content can drop under the 12% border, resulting in chromium depleted zones wich are very sensitive to corrosion. This zones also have to be removed ! And it is only the pickling proces that can do this in an effective manner.
So here's my conclusion : after welding, oxide layer, discolouration and chromium depleted material have to be removed in order to restore corrosion resistance properties of the base material. Pickling removes everything. Mechanical methods do not remove chromium depleted zones. That is why I mentioned in previous posts the difference between "critical" and "non critical" corrosion environments.
So watch out with stainless steel for storage tanks, vessels and piping. Treat it like a baby.
Further to Mr. Buyle's answer, with which I agree in part, I'd like to inform you that one of the last issues of Welding Journal, the AWS monthly magazine (I don't remember if it was the July or August issue), dedicates a full article, with plenty of explanations and photos, on how to get rid of colored layers around s.s. welds.
I don't agree with Mr. Buyle's opinion in that blasting is OK for structural steel. Stainless steel is too soft to be treated with sand (hardness 7 in Mohs' scale) or garnet (hardness 8 in the same scale). At the most, pecan shell could be admitted.
Giovanni S. Crisi
Sao Paulo - Brazil
I am also trying to learn the finer points of working with stainless steel and how to treat it properly.
In the process, I came across a company that may be a useful resource for you. Their name is Intercon Enterprises Inc. web is www.intercononline.com and tele. is 800 665-6655. They sent me a small packet that explained their product and was informative to someone with little or no knowledge of dealing with cleaning ss.
Ken
Forum
