Electropolish vs. Pickle and Passivate?

[Gungadin]

Howdy folks - I'm hoping to get a basic understanding of the difference between electropolishing and pickling/passivating. I have a number of SS louvres (4' x 1'6", 316 SS) which are installed in a water tunnel portal building, and exposed to high humidity from slightly chlorinated water. The louvres exhibit general surface corrosion and corrosion on the welds, and the plant operators want to have them cleaned up. So, what's the best way to go?

Thanks,
John

 

[mcguire]

Both processes basically do the same thing, i.e. selectively dissolve free iron, oxides, exposed inclusions...anything that's not stainless steel..from the surface. Electropolishing goes one better by smoothing the surface on a microscopic level so that it is more crevice free and free of damaged metal.
You will get optimum corrosion resistance from electropolish.

 

[Gungadin]

Thanks McGuire - I read in another thread (338-65804, "pickling and passivating)that wire brushing/mecahnically abrading the SS surface can have a detrimental effect on corrosion resistance - if so, is that alleviated by subsequent electropolishing?

Thanks again,
John

 

[mcguire]

Yes, it is, if all the disturbed metal is removed by the electropolish. This is normally the case.

 

[unclesyd]

Neither process is a permanent fix if you have chlorides present. It will not take long for the metal to be back in the same condition as you started with. One thing after you get them back to a better appearance a periodic washing will help. We use a periodic wash with one of the commercial H3PO4 Rust Removers or Wash Primers, Wedac, Bull Frog, at 100% concentration to clean stainless steel. It does a very good job and help slows the corrosion down.
We use fine Scotch-Brite pads with the appropriate protective gear, rubber gloves, apron, face shield, etc.

Electroplishing of large parts is expensive due to power required. Pickling and passivating have their pit falls, mainly handling of the acids and disposal of same.

 

[mcguire]

unclesyd's experience is a wealth. I caution that Scotch-brite is an abrasive and one must use the FINE that unclesyd recommends or you'll screw up the stainless as badly as if you had gotten a #3 polish.
The resistance of stainless to chlorides is accurately predicted by the PREN formula (see FAQ's), as long as you're not dealing with welds or abraded parts.
A very strong pickling is as close to electropolish as one can easily, and cheaply, do.

 

[unclesyd]

If you do pickle with HNO3+HF keep the HNO3 to HF ratio high. I would use 20% HNO3 + 2% HF at 60°F to 80°F. You might have to go up a little with the HF depending on amount of corrosion products.
If I passivated I would use 20% HNO3 at the same temperature.

Watch yourself and others with the HF, its very nasty stuff. HNO3 is too, but you do grow back.
Take all the necessary precautions for handling these material.

mcguire,
We use this method to clean and polish laboratory fume hoods among a lot of other things. Using a fine grit we see no problem with the finish.
We do coat the stainless with a commercial product called Shelia Shine, don't know the ingrediants but it does a pretty good job keeping the stainless bright.

 

[mcguire]

unclesyd
I don't think anyone has measured the decrease in corrosion resistance as a function of surface roughness or some other quantitative parameter, but I do know that taking an electropolish finish to 10 rms by abrasion lowers the critcal pitting temperature from 20C to 0C.
Based on this experimental data, I don't trust any abrasion as harmless. I may seem extreme in this, but I think abrasion has no useful role in stainless except to remove oxide and is then followed by pickling or EP.

 

[unclesyd]

mcguire,
You maybe correct in respect to abrasion in that situation. But haven’t you found that a lot of this data doesn’t pan out in the real world because of very slight changes in nearly everything associated with the application. There was an awaful lot of work on surface finish done and published in some earlier books on corrosion.

We did an awful lot of work with 316 S/S plate that we used in making spinneretts. We worked with surface finishing, cleaning methods, surface treatments, etc. We really didn't have a movement in corrosion rates of 316 S/S until we got above 64 RMS (we wanted less than 2 rms) with our processes.

All our corrosion test coupons, lab or field. used were 180 grit finished. For many years we kept 2-3 Huey test, mainly 304L, going and as you would expect the finish did affect the first and sometimes the second cycle as the materials were corroding, but it leveled out quickly.

Electroplishing is expensive and you can electropolish some pretty rough surfaces. This is also true of mechanical polishing.
The process people wanted the flattest/smooth finished possible.
We proved this many times as with our spinnerette manufacturing process, the flatter (< 1/4 light band) we got the finish turned dull and looked matte. This wouldn't sell until we polished it. We could polish 8-10 RMS and sell it.

I've mention this before but for 20 years all S/S pipe and plate were possible was pickled with HNO3/HF and passivated
with HNO3. Most of the time we only pickled and didn't passivate. This was all abandoned when we demonstrated that material going into a corrosive process all the extras didn't count. This is in an industrial not pharmaceutical plant where a little iron or Cr green want hurt.
One reason I did a lot of work on cleaning is we had thousands of square feet of S/S in air handling units that required periodic cleaning. I found that the full strength commercial H3PO4 materials sprayed or wiped, rag or Scotch-Brite, allowed to sit and rinsed off gave the best overall results, both in cleaning and safety, with S/S.
I just like to try the less hazardous materials first.

 

[Gungadin]

UncleSyd - is the Shelia Shine a clear coat protective finish suitable for outdoor exposure? What's your experience with its longevity?

Thanks!

 

[mcguire]

unclesyd
My real world experience was in the use of flat rolled stainless for appliances, commercial cooking equipment, architecture, and general pipe and tube. We found that 2b, 2d, and bright annealed finishes all were nearly optimum. Electropolished would be a slight improvement. The big change came when we polished the surface. A #3 or a #4 finish ( 5 to 30 Ra, 120 or 180 grit) decreased corrosion resistance in service greatly. The amount was equivalent from the loss of 5% chromium, more than the difference between 304 and 316.
Smart customers, upon learning this,swithched from 304 polished to 201 or 430 with a simulated polished finish made by rolling bright annealed stainless through a temper mill with specially engraved rolls. It looked the same as polish and was more reproducable. With nickel at $6/lb they saved big time and upgraded their product.
I'm continually amazed that people specify 304 when they could use low nickle 201, but I shouldn't be. The service centers and the nickel people really fight against 201.

 

[unclesyd]

Gungadin,
We only used it indoors in different environments. I understand it was developed for use on with all the S/S railcar type dinners that were quite popular several years back.
I'll try to get some information on it from my supplier as I don't have a label.

There is another product called Semichrome that polishes and protects S/S and Aluminum that does work very well outdoors. Also I have use several marine products some appeared to work and some didn't.

mcguire,
Agree completely with you observations. We did see specific break points as I mentioned. I have found if the corrosion rate is above 2-3 mpy the surface finish only plays a small roll on the corrosion rate and that is in the initiation. When the rates were higher we saw no difference.

One reason 304 S/S is usually specified is that it's known and available. Quite a few years ago you heard 18/8 a lot. In the US it's amazing how much a sales pitch affects the request for materials. A few years ago a pitch came out for &quot;Supertainum&quot; cookware. I saw it called out on 2 prints.

I fought the sales representatives and service centers for many years. This of course changed with the markets, buyer's market or seller's market. It is definitely a seller's market now.