Stainless in contact with carbon steels

Date 08-05-2005 13:07

I think the answer has less to do with the actual potential for cross contamination (in your specified situation). I think it goes to the mindset that you want to instill in all parties. If they are allowed to treat some situations casually, there will come a time when they treat the wrong situation casually and it will cost you.

Stainless should be segregated from carbon steel at all times. A work bench used with carbon steel materials will have trace levels of iron on it that could become imbedded in the stainless (however insignificant that level of contamination may be).

I know this is the level of expectation from customers who pay the extra money for stainless; but I also know it is rarely adhered to (our facilities included).

Charles

By - Date 08-05-2005 13:14

Hi Jon,
Very good topic for a weekend of discussion. Since you asked for opinions, please allow me to give mine.
In your example of laying a stainless steel on a carbon steel workbench, well, it will be detrimental ONLY if it destroys the chromium layer of the SS and impregnates carbon into the SS. Usually, the SS is scooted around on the CS table because it is there to do some fabrication or something that involves moving the SS around. The secret is not to destroy the invisible, impervious layer of chromium, which is where SS gets it's surface corossion resistance. Long term exposure, and under certain humid temperatures, to CS can definitely be detrimental. But, even under short term contact to the CS workbench, once the SS is removed it should be cleaned on the side that was laying on the table to remove any contaminants picked up from the table.

Chuck

By jon20013 (*****)

Date 08-05-2005 13:19

Thanks guys! So, Chuck, in your opinion, if I were to get dinged for violating a taboo such as defined above (short term, casual contact) I could probably appease those with concern by simpling applying an acid wash pickling to the parts?

By - Date 08-05-2005 13:40

Jon,
Working a nuclear environment is usually a more stringent set of rules, as you and I both have experienced. Applying a pickling treatment will only benefit you if you have destroyed the passive layer due to contaminated contact. It is sort of like a false sense of security. If you are sure you have not impregnated SS with CS contaminents, I don't see where a pickling procedure is going to have any significant advantages. In this case, it is educating the people that have this false impression. I mean, another pickling process is not going to hurt the SS, and if it will appease the higher-ups, then it might be the way to go. But, realizing exactly how SS is affected, and how corrosion starts in SS is of equal importance. As Charles said, it is the general opinion that regardless of the exposure to CS, the SS will be affected. But, in reality, it is only if the chromium layer is compromised allowing the carbon to impregnate into the unprotected surface of the SS. The general feeling in the nuclear industry is to not take any chances, regardless how small. Of course, telling you that is like preaching to the choir.

Chuck

By NDTIII

Date 08-07-2005 03:21

I work inthe Oil & Gas inustry in the Middle East and even here we do not allow s/s to come into contact with c/s, unless it is a dissimilar weld application.
If tools are used on carbon steel they cannot be used on stainless. The stainless tools are marked with blue paint. If we even se them in the carbon steel shop they are trashed. Same thing goes for c/s tools in the stainless shop.

By billvanderhoof (****)

Date 08-09-2005 06:36

Conceding that contact between ss and cs is detrimental to the ss. Is the opposite also true, that is is there some detriment to the cs?
Bill

By NEQA

Date 08-08-2005 15:37

Allowing SS to contact CS is a no-no. Doesn't matter for how long. Which is why, when we buy SS, we make our source ferroxyl the SS. If the material is to be rolled, it is ferroxyled at the roller, and again when it returns to the shop. It is ferroxyled again before AND after bead blasting as well.

All of our Contracts have a separate spec. on SS. It tells our supplier how we expect them to handle, store, work with SS - and our inspector strictly enforces all the provisions of the SS spec.

Our experience (non-nuclear) is that even casual, short term contact in "normal" temperatures and humidity levels can contaminate SS.

By TimGary (****)

Date 08-08-2005 15:52

Could you please define "ferroxyled"?

Thank you,
Tim

By - Date 08-08-2005 17:34

Tim,
A Ferroxyl Test for Free Iron, outlined in ASTM A 380-99, section 7.3.4, is a very highly sensitive test and should be used only when traces of free iron or iron oxide is "objectionable". It should be used only by personnel familiar with its limitations. It consists of 94 weight % of distilled water, 3 weight % of nitric acid, and 3 weight % of postssium ferricyanide. The potassium ferricyanie can emit lethal fumes if they are exposed to concentrated acids, or heated to decomposition. Highly trained personnel should only be permitted to perform this test. The preferred method of application is by atomizer spray, and inhalation of this spray should be avoided. Carbon steel will only contaminate the stainless steel if the passive layer of chromium is destroyed. Very few fabricators have the capability to fabricate in an operating room environment. There are many ways to remove free iron and free iron oxides by means other than a ferroxyl test. Actually, a ferroxyl test is a test done only to "detect" contamination. Once contamination is detected, then a removal application is done.
Iron contamination on stainless steel is almost always confined to the surface. If reasonable care is taken during fabrication, simple inexpensive cleaning procedures may suffice for its removal, and very little special cleaning would be required, unlike the ferroxyl test. Personally, doing a ferroxyl test after every fabrication process is not necessary, but whatever...

Chuck

By TimGary (****)

Date 08-08-2005 17:52

Thanks Chuck!
Fascinating stuff....never heard of it before.
So I take it that the Ferroxyl test leaves a stain or other detectable trace over CS contamination on the SS?

I'm thinking that this test would be useful in SS pharmaceutical equipment applications, for example the ultra pure steam generators that I used to be involved with.

On a somewhat similar topic, I was wondering if you would be so kind as to shed some light on the "rouging" phenomenon that sometimes happens to SS. Could this ferroxyl test help in determining the potential for delayed rouging to occur? Do you know of a method to eliminate rouging other that passivation?

Thanks for your time,
Tim Gary

By - Date 08-08-2005 18:04

Hi Tim,
A blue stain is left if there is indications of surface iron contamination (within 15 seconds). It may take up to several minutes for the detection of oxide scale.
Actually this test is not recommended for food-processing applications, beverages, pharmaceuticals or other products for human consumption unless ALL TRACES of the solution is sure to be removed. You have got to be extremely careful when using this test...

Chuck

By - Date 08-08-2005 18:29

Tim,
Sorry, I didn't address the "rouging" part of your question. I hit the "post" button before I got to that part.
Rouging, by definition, is the result of the formation of iron oxides, hydroxides, or carbonates either from external surfaces or from the destruction of the passive layer of the stainless steel. Depending on severity, the colors can be from orange to red to black. Either way, it is an idication of surface contamination which, when left, can cause a possibility of formation of intergranular corrosion. Now you've got a real problem. Surface contamination, when detected, can be resolved. Intergranular corrosion, in most cases, goes undetected until a catastrophic faliure occurs. Then it's too late. That is why it is so important to make sure the invisible, impervious layer of chromium is not compromised on the surface of the stainless steel. I mean, in the real world, (refineries, nukes, power houses, paper mills, etc.) stainless steel comes in contact with carbon residue all day long, but it is only detrimental if the passive layer of the SS has been destroyed, then those particles can act aggressively on the unprotected SS surface.
The ferroxyl test is not used to determine the potential for "rouging" or any other contamination potential. It is used only to determine if there are any contaminants on the surface of the SS during the test. Nothing more. With a ferroxyl test, the solution should be removed as quickly as possible as to not compromise the surface of the SS. In my 35 years of dealing in stainless steel, the ferroxyl test is only used when there is an objectionable agreement/disagreement of free iron or iron oxide contamination. If there was any doubt that free iron or iron oxides exist, the surface of the SS should be cleaned without the implementation of a ferroxyl test. Again, that's what I've experienced over the years.

Chuck

By TimGary (****)

Date 08-08-2005 21:15

Thanks Chuck, you hit it with a spotlight!

Thanks for your time,
Tim

By NEQA

Date 08-08-2005 20:33

We will require a spot ferroxyl test only on those areas we suspect might be contaminated - the end of a plate is a common problem area. Especially if the plate has been laying on the ground, uncovered, for any length of time. It is a quick and very reliable method for detecting a potential problem. It can also be brushed on.

By - Date 08-08-2005 21:04

According to the ASTM, "The test solution will change color on standing and must be mixed fresh prior to each use." You guys must stay busy just mixing chemicals. :-)

Chuck

By - Date 08-08-2005 21:29

NEQA,
Brushing the ferroxyl solution can possibly give an inaccurate result. When brushing is done, the microcrevices can easily be missed. The only acceptable application, according to ASTM, is with an aluminum, plastic, glass, or rubber atomizer having no iron or steel parts, or a swab (atomizer spray is preferred). I am not criticizing your application method, just trying to give you accurate information based on ASTM A 380-99. Unfortunately, or fortunately, when doing a ferroxyl test, it probably should be done according to the procedure set forth in ASTM for best results. Not trying to intervene, just help.

Chuck

By NEQA

Date 08-09-2005 12:32

Chuck - good advice. You are right A380 doesn't recommend brushing the solution on, but there is more than one way to skin a cat. I also didn't mean to suggest we ferroxyl every piece of SS we buy. We require it usually only when we have reason to suspect trouble. A lack of receiving inspection, how/where the SS is stored, etc. are pretty good clues as to whether we will have problems with contamination.

The solution takes perhaps 1-2 minutes to mix, and 15 seconds to tell us if there is contamination. It is not a labor intensive operation.

By TimGary (****)

Date 08-09-2005 15:54

NEQA,

Just out of curiosity, I was wondering for what application are you using SS that requires such stringent inspection requirements and what safety precautions do you take?

Tim (curious george)

By NEQA

Date 08-11-2005 18:09

Tim:

We are the Owner of a very large utility

By - Date 08-09-2005 19:24

NEQA,
Please, I was not criticizing your inspection methods. Sorry if it came across as that. I only meant to say that brushing can actually not penetrate some crevices where corrosion starts. That's all. A nitric solution (passivating process) is what I see happen more times than a ferroxyl test. Hey, if it works, then it ain't broke. :-)

Chuck