stain-less steel or rustrproof steel ?

[zardoz]

i see this guy on ebay selling an exhasut. it says it is stainless steel but it has rust on it. I asked him and he says stainless steel just means "stain-less and not rust proof"

ebay link

Surely the passivation layer on the steel prevents all rust forming? Meaning the only rust on it must come off the hangers/brackets etc. I think he has a galvanized exhaust. What do you guys think ?

PS: not that im any expert on stainless steel. I only found out a few months back (thanks to this forum) that some stainless is magnetic. However i could not get a local scrapyard dolt to accept a load of hose couplings as magnetic stainless as he didnt belive it exsisted

 

[malcolm]

Looks like a bog standard exhaust with an aluminised coating. Stainless ones tend to look more yellow once they've been hot. Suspect the eBay dude doesn't know what stainless is.

Here's a bad picture showing a used stainless exhaust (to the left).

 

[wyn]

Looks like a bog standard exhaust with an aluminised coating. Stainless ones tend to look more yellow once they've been hot. Suspect the eBay dude doesn't know what stainless is.

Here's a bad picture showing a used stainless exhaust (to the left).

like malcom says , no way thats stainless. Where the front box has got hot the coating is burnt off the mild steel. It also has that dull silver colour of an aluminised coating.

 

[rtbcomp]

From what I've seen there are two type of stainless used in exhaust systems. Nice shiny stuff for silencers which are fabricated and duller stuff for the pipes, which is easier to bend.

The dull stuff will accumulate a light coating of rust, which should NOT be removed. Certainly you shouldn't get the red rust like you do on mild steel.

 

[rtbcomp]

PS: not that im any expert on stainless steel. I only found out a few months back (thanks to this forum) that some stainless is magnetic. However i could not get a local scrapyard dolt to accept a load of hose couplings as magnetic stainless as he didnt belive it exsisted

My local scrappy calles magnetic stainless "Chrome Steel" and he only gives the same as mild steel.

 

[COMP]

some SS will get rusty

 

[zardoz]

here is a better bigger pic of the offending item

 

[malcolm]

Is it even possible to form a stainless silencer with rolled edges like that. Would have thought stainless would work harden and split.

 

[Wozzaaah]

I'm sure that's just a standard aluminised exhaust Zardoz, it just doesn't look like a stainless one at all.
All the stainless ones I've ever seen have looked like the one in Malcolm's pic.(I spent 8 years working in a tyre and exhaust place so I've seen a few in my time )

 

[fingers99]

To me, that looks galvanised (the white/grey is a give away).

Some grades of stainless will rust, some will remain maleable enough for rolled edges.

Exhaust manufacturers do all kinds of cranky stuff (use wrong grades, weld mild steel to stainless, galvanise and aluminise stuff, or make stainless and mild steel exhausts to the same pattern).

Tis a nightmare telling what's what!

 

[zardoz]

Yep galvanized was my first thought. i learned to tell galv from ally or stainless, at a glance, a long time ago. It saved many a wasted journey to the scrapyard

 

[Wozzaaah]

If you want a bespoke one made up, why not ask Malcolm .

Were you ever a Blue Peter presenter Malcolm?

Sorry, googled 'stainless steel silencer' and found that. Here's the Full link

 

[snowcat]

austenitic S/S can become magnetic using certain fillers IE 316Lsi can leave magnetic deposits, I believe S/S exhausts use 304 to help cope with sulpher. Corrosion is formed by chrome carbides and can be quite severe in certain enviroments...

Yes your quite right S/S will corrode, and the wrong choice of grade consumables welding temperatures only makes matters worse... high temperatures in use will also make it more inclined to "spot"

 

[Knipex]

All grades of SS will rust (well pretty much all grades) unless passivated. Even then if welded or sanded they will rust unless these areas are repassivated.

Passivation is not a complicated process but depends on the base metal is it 303 \ 304 etc. If 304 then get it clean (I mean really clean not half clean) and sit it in 20 to 30% Nitric acid for a few hours. If its 204 you need Dicromate (which I would not recomend unless you are srt up to use it) or a citric passivator such as citrisurf 3050.

There are lods of places all over the UK who will passivate these.


That exhaust is not stainless its mild steel aluminised.

 

[Paul.]

All grades of SS will rust (well pretty much all grades) unless passivated. Even then if welded or sanded they will rust unless these areas are repassivated.

Passivation is not a complicated process but depends on the base metal is it 303 \ 304 etc. If 304 then get it clean (I mean really clean not half clean) and sit it in 20 to 30% Nitric acid for a few hours. If its 204 you need Dicromate (which I would not recomend unless you are srt up to use it) or a citric passivator such as citrisurf 3050.

There are lods of places all over the UK who will passivate these.


That exhaust is not stainless its mild steel aluminised.

I think were getting a bit carried away here, passivation of stainless is carried out, but usually on precision or critical components such as aerospace or similar, surface corrosion of stainless steel is usually caused by contamination from tooling, close contact with steel benches vices etc, or is sometimes seen in welds due to sulphides which are added to stainless to aid machinability, the usual fab shop method of dealing with this is to use a pickling paste or manual cleaning /polishing / glass bead blasting etc to remove surface contaminants and to allow the residual oxide to reform naturally, virtually everything that I make is for the processed meat industry and is permanently wet, and I have never had a job go rusty yet

 

[Knipex]

Paul

What Grade of Stainless do you normally use ? I had a guy in here last month in serious trouble. He had made a number of support units for a large pharmaceutical company from 304 stainless and they were all rusting. They were used to support tray on which electrical cables and pipework were ran from one building to the next so they are outside bot sheltered from the worst of the weather.

As they were all permanent structures it looked like he was going to have to scrap them and make new ones and wanted us to passivate them prior to being erected. We ended up saving the old ones by manual polishing and passivating using a citric based Gell. Took 4 guys 2 weeks to do the work and cost him a fortune.

I will admit his main issue was that 99% of the work he did was in mildsteel so he had a lot of contamination from bad handling. The only grade of Stainless I wouldn't passivate is 316. In the long run its safer and with citric based passivators available relatively cheaply for self application there is no reason not to.

 

[Weldingartisan]

Sorry Knipex, but I agree with Paul. As long as you don't cross contaminate and use the right abrasives/polishes passivation is not always necessary. The reason not to? Cost for one, it's another process.
Back on the thread, I don't think many commercial manufactures even use 304 on thier stainless exhaust systems, judging by some of the cr*p people have asked me to modify!

 

[Paul.]

Paul

What Grade of Stainless do you normally use ? I.

Mostly 304, sometimes 316, all welding consumables are 316 regardless,
Its inappropriate for me to comment on your particular customer, but stainless steel fabrication does require a slightly different approach to mild steel, if the 2 processes are carried out in the same shop there will always be some cross contamination from grinding dust etc, I have seen mild steel wire brushes accidentally used to clean stainless welds, and the welds then go rusty, and even different materials stored together can give you contamination problems.
I think I would have cleaned your customers gantries by glass bead blasting, I have used this process on many machines that Ive built over the years without any problems at all,

 

[StainlessBloke]

Hey all. I know this post has been dormant for a while, but I just wanted to clear up a few points raised about the treatment of stainless steel in previous posts. In keeping with the non-advertising ethic of the forum, can I just say that I work for a firm who work closely with ss and no more; thus the the information is offered in good faith!

ALL stainless steels can rust, without exception: it entirely depends on the environment they are to be used in. Some are more resistant to certain chemicals etc. than others. It's surprising how many people genuinely believe that this metal is rust proof which is absolutely not the case. If the chromium layer is damaged by mechanical or chemical attack, then the metal is susceptible to corrosion, end of story.

Passivation as a technique, is used by certain specialised industries,as mentioned above, as well as in a lot of medical/pharmaceutical and increasingly catering components/environments. It is absolutely true to state that the passive layer (which gives the steel its corrosion resistant properties) will form itself as long as the surface is clean, intact and exposed to air - artificial passivation will however, accelerate this process and there is also evidence to demonstrate that not only does the induced layer form more quickly, but it also remains a better barrier permanently.

Its always worth bearing in mind when working with stainless, that anything you do to the surface of the steel is going to be to the detriment of the passive layer - using scotch bright, manual polishing, welding, etc. - all these processes damage the surface and inhibit the formation of the chromium layer. For ss to be as corrosion resistant as possible, it's generally accepted that the best process for the material to undergo is electropolishing, as unlike manual polishing, this process actually removes a layer of metal and doesn't entrap any particulates in/below the surface. Hence it is widely used on ss components which are placed in the human body during surgery and the like.

Not trying to be smarmy or anything guys, just thought this info might be useful.

Cheers.

 

[fingers99]

Stailess Bloke, what exactly is electropolishing?