stainless steel and ozonation 1

[schick]

I need a little help in determining what I need to ask to determine the nature of a problem one of my customer's is having.

We have a piece 316 stainless tube for a valve stem which passes through a 304 stainless guide, and has a plastic cap on one end.

As a method of cleaning, the customer uses ozonation(sp?).

The problem is that anywhere there is any type of mechanical contact, there appears to be some scoring of the parts, as well as a yellowish discoloration, almost as if the parts are being heated.

Since the product is drinking water, there can be no other lubrication than the product itself.

Is anyone familiar with the ozonation process? is there some kind of particulate which is created with either 304, or 316?

 

[mcguire]

it appears that there are two phenomena going on. One sounds like galling. That is the unlubricated frictional contact between two stainless surfaces which results in metal to metal micro-welds and rapid wear.
The yellow discoloration you associate with the ozone treatment sounds like it's producing an oxidation. Stainless usually discolors to a yellow oxide around 600F. Maybe with ozone that occurs at a lower temperature.

 

[aerstream]

I too have seen the build up of yellow/brown deposits on 304 because of ozone/corona discharges.

Do either of you (or anyone else) know whether 316 would be more/less resistant to this oxidation or just the same?

Thanks for any tips in advance.

 

[mcguire]

Since the governing phenomena appears to be oxidation rather than corrosion ( i.e. failure of the passive layer) there's not much reason to believe that 316 would do better.
It may do worse since it has less chromium.
To minimize this condition you would need a more oxidation resistant grade, which is not necessarily the same as a more corrosion resistant grade. 302B, 309, or 310 would work if you can obtain these less frequently used alloys.
They are used for heating elements, furnace parts, etc.

 

[aerstream]

Thanks for the advice on the steel grades, I'll look into them straight away.

I have also found that the oxidation products are quite tenatious. Cleaning the metal completely is difficult, and oils and common solvents don't really help.

My searches indicate that some sort of acid might be helpful, but I'd rather not use acids. I don't suppose you know of any surfactants/detergents that might help separate the oxide from the metal?

It's a long-shot!!

Cheers.

 

[schick]

316 definately doesn't help....That's what we found the problem on.

as far as detergents, etc. I haven't heard of anything.
Is your part something which can be hit with a fine wire brush? That might help clean it off, mechanically, but you'd probably have to passivate after that.

 

[mcguire]

Aerstream and schick
The methods to remove heat tint on stainless are given in the FAQ's at

http://www.stainlesscenter.com.

You need to register there, but it's free. Look under "clean stainless" and then "weld". The appropriate entry is about heat tint. There are several methods, using mechanical methods, phosphoric acid, or electrolytic.
The condition is similar to that encountered after welding when the heat affected zone gets various levels of heat tint that must be removed to restore corrosion resistance.
Since the problem is galling, I would recommend the abrasive methods since it will result in a superficial surface hardening. I have seen this improve galling resistance.

 

[bilge]

The superficial hardening is likely caused by stress-induced transformation of the surface austenite to martensite. If that is so, a stainless with higher carbon content should have a harder (and more galling resistant) surface than one with a lower carbon content.

 

[mcguire]

bilge has a good point and this would lead one to consider 440C which is the hardest, highest carbon stainless. It will require much different processing than an austenitic, but that may be required.

 

[aerstream]

Thanks for all your help, everyone.

The stainless part is quite specialised - a piece of capillary tubing - and although I've asked our manufacturers if they make it in other grades (such as those suggested) they have not yet responded.

In the meantime I'm running a few in-house tests (accelerated corrosion then various cleaning methods). I'll post any interesting results.

Thanks again!