Pitted Stainless Steel Brake Pistons - or Plated?

[david.g.chapman]

Yes, that's right!

I has a nasty shock when I took my 7 year old pistons out of the rear calipers, I was thinking all they needed were new seals. There was virtually no wear on the pistons, but each one has one or two pits about 1mm across and deep enough to feel with your finger nail. The pistons were not leaking.

I suppose the pits are down to inclusions in the steel creating a corrosion point - but I was still disappointed.

Although the pits are narrower then the width of the seal, I suppose I should replace the pistons. Filling the pits with something like JB Weld sounds too iffy for me.

Any thoughts on which ones are best, plated steel, stainless steel, or some other fancy material like an alloy? I am not sure any more.

[terryp]

David
I have the same thing on my Caterham fronts, tried plated last time and they lasted about 3 years, stainless next time!

Terry

[alan.barker]

on my Elan Sprint rear calipers i fitted stainless from a supplier in the USA
Alan

[RichardHawkins]

David,

Corrosion can be a very difficult and interesting problem. Like you I would have not really thought about stainless brake pistons, and assumed they would be a fit and forget option. However, your observation reminds me of something my metallurgy lecturer demonstrated 40 plus years ago. He placed two pieces of stainless in a Pyrex cylinder of salt water (just mildly corrosive) on one he stretched an elastic band. By our next lecture it was obvious that corrosion had taken place under the elastic band. This little experiment was to show us that the reason stainless resists corrosion is that there is a passive film of chromium oxide on the surface of stainless steel that is being continuously repaired (if damaged) by oxygen in the environment. The rubber band had prevented access by any oxygen. This might be the reason the brake pistons have corroded.

This might be the reason motor vehicle manufacturers use chrome plate (I assumed it was just cost). There are many different grades of stainless, and perhaps you have been supplied with something unsuitable.

I worked in the chemical industry, and was often amazed by what corroded and what did not.

What we really need is a metalurgist or material scientist within our group to comment. Failing that the brake fluid manufacturer may be able to help.

Richard Hawkins

[rgh0]

Yes "crevice corrosion" is a common feature in stainless steel in presence of things like chlorine ions from salt. It happens under gaskets or seals where pockets of corrosive ions can build up and the chromium oxide layer on the stainless gets depleted allowing corrosion to occur. Typically is appears as deep pits like you have.

Titanium or duplex stainless steels are more resistant to it.

cheers
Rohan

[david.g.chapman]

I tried filing into an area around a pit, and was surprised when the file started biting into the steel. When I stopped there was a colour change around the pit as if I had filed through a plated layer.

Looking through my old invoices I found the one for the pistons - no mention of stainless steel. I can only assume that my memory must be playing tricks, because I was sure these pistons were stainless steel. They certainly look like stainless, and not chrome or bright nickel plating.

Can anyone confirm that slainless steel pistons are solid stainless steel. if so, my path is clear and stainless is the way to go after all.


Dave Chapman.

[Esprit2]

Stainless Steel isn't just one steel. There are many "stainless" alloys, and some are way better than others. Kitchen flatware is typically made of 416, and it's only marginally stainless. I once put some "Iron Out" into a dishwasher to remove some minor accumulated stains due to iron in the water. But I did it in a normal wash cycle with dishes in the washer. All the glass & plastic bits came out squeaky clean and bright, and all the stainless steel flatware was solid brown with rust... it was 416 stainless.

18-8 / 302 are much more 'stainless' than 416, but not necessarily the best.

The stainless caliper pistons are all aftermarket, not "Girling", and It would be interesting to know from which stainless yours were made.

Regards,
Tim Engel

[john.p.clegg]

"on my Elan Sprint rear calipers i fitted stainless from a supplier in the USA
Alan"

looks like we were of one mind,i bought a couple of sets ( always carry a spare ) to rebuild mine this winter..

John

[UAB807F]

David,
What we really need is a metalurgist or material scientist within our group to comment. Failing that the brake fluid manufacturer may be able to help.
Richard Hawkins

If you can post a photo of what you've got I'll have a shot at it. I'm still technically a metallurgist although it's been a while since I did any serious failure investigation. (early retirement )

I wouldn't have expected stainless to pit under these conditions although I suppose it's possible. The cheaper stainless steels with basically steel & 10-12% Cr will probably be the most susceptible - easily spotted as they are magnetic. What's often referred to as 18/8 (18Cr, 8Ni) or 300 series steels (non-magnetic) are much better although the basic alloys still get this type of corrosion. To resist pitting the common method used to be to add 2-3% Mo although there are other options - higher Cr, Ni, etc.

On the question of "are stainless pistons are solid ?" then yes, they will be and most likely machined from bar stock. That's one potential weakness, the better machining grades aren't always the best for corrosion resistance although in this application I'd have thought them ok.

If it's any interest, I fitted stainless pistons back in 78 or 79 and the same ones are present. I've changed the rubber seals a few times as they seem to absorb & swell from brake fluid and the action isn't as crisp, but the same pistons remain, in fact it's only this year I did the seal change routine again.

Brian

[alan.barker]

that sounds good seal change routine every 35 years,
I fitted the stainless pistons on my Sprint about 10 years ago so that means i have another 25 years to go
Alan

[UAB807F]

Even I can't get 35yrs out of a set of brake seals Alan, and the missus always compares me to Scrooge when it comes to spending cash.

I'm surprised you've got 10yrs and counting. I can't honestly say I've measured but my guess would have been 3-5yrs, and this year's set was probably at the 3yr mark. That was unexpected but at this year's MoT the guy mentioned he'd noted a disparity across the rear axle on the footbrake and although it was a pass, perhaps I'd like to look at it. I couldn't tell anything from driving and neither side was sticking but I changed them all the same and the seals were quite swollen compared with the new set.

Brian

[david.g.chapman]

Thanks for all your replies.

I soaked a piston in brine overnight, and the filed patch has started to rust. Looking carefully at the inside of the piston there is roughness there, so I believe I have cast pistons that have been turned in a lathe and then plated in some harder material.

I suppose one test is to look at the inside cavity to see if is a machined surface, indicating machining from solid.

So its off to the stainless steel piston shop for me.

I will change the title of this post a bit to avoid confusion if someone searches in future.

Cheers

Dave Chapman.

[vstibbard]

I' always opt for plated stainless steel as the best long term solution with the plating to provide a harder surface than stainless which avoids nicks, etc

V

[PeterK]

I fitted plated pistons (not chrome) rather than stainless, sourced from a well known automotive supplier (not Lotus specialist). About 6 months later, when I had started to drive the car, I noticed that the brake fluid was dropping. Turned out that the plating was faulty, and the pistons were very corroded.
Supplier was very apologetic, and rather than replacing pistons, seals and pads, I paid a tiny amount more and got a pair of new callipers.
Next time I rebuild callipers, I will probably use stainless.
Peter

[gus]

Stainless rear pistons 1985
Stainless front pistons 1988

Silicone fluid

Never been a problem since

It is however important to occasionally replace the fluid, as I did have a master fail from water inclusion over a winter.