Cleaning Exhaust Headers With Harpic Max Strength

I’ve been looking at how to clean the brown ‘stainless’ exhaust headers on my Triumph, and there are lots of people on the Web recommending the use of Harpic Max Strength bleach. This seems to work because it contains hydrochloric acid (aka muriatic acid for any non-Brits out there) – but is there a down-side? I’ve done a little research, but feel sure that there must be someone out there with metallurgy expertise.

The science seems to be as follows:

The Scientific American web site says that stainless steel contains iron, chromium, manganese, silicon, carbon and, in many cases, significant amounts of nickel and molybdenum. These elements react with oxygen from water and air to form a very thin, stable film that consists of such corrosion products as metal oxides and hydroxides. This is often referred to as ‘passivation’.

I can’t find a definitive description of the staining cause, but it seems that the brown colour that we see is this ‘passive’ film (which protects the steel from further corrosion) being stained through the heat and chemicals that it is subjected to.

Given the above, it seems very likely that if the acid is getting rid of the brown staining, it is probably removing the passivating layer that protects the steel. That explains why it doesn’t take any elbow-grease, which trying to achieve the same effect mechanically using an abrasive polish, such as Solvol Autosol, would take.

So – I did a search to see whether hydrochloric acid is harmful to stainless, and it does seem to be. Most sites warn against using it – just take a look.

I figure (hope?) that efficient removal of the acid once it’s worked, ought to prevent any adverse impact of using the acid/bleach on the headers. I just sloshed on plenty of water and dried everything carefully. This combined with a good polish with my trusty Solvol Autosol polish certainly returned my headers to almost-new condition. I think it’s wise to give the pipes some time to re-generate the passivated layer before exposing them to corrosive salt (or whatever), and this is recommended on some web resources.

If this approach is ok, then it would be an attractive option, as this bleach is readily available (it’s in UK supermarkets) and it’s very cheapI know that there are plenty of alternatives. I’ve seen recommended:

• Welding ‘pickling solution’, but I don’t know what this has in it, and obviously it isn’t as easily available. It might have nitric acid in it, which seems to be ‘stainless-friendly. (don’t know if this works)
  
  
• Oven shelf cleaner kits - i.e. a bottle of oven cleaning gunk and a big plastic bag. This needs you to remove the pipework – and it didn’t work on my Ducati exhaust – made no difference at all.
  
  
• Elbow-grease and Autosol (there’s a Dellboy’s garage YouTube video) – seems to work if the tarnishing is light, but it didn’t work on my Ducati.
  
  
• Full-on bench polishing machine with mops and polishing soap – expensive, but it’s bound to work, I suppose.
  
  
• I’m sure that there are some proprietary products, but they’ll be expensive, so if the Harpic trick works…
  

So – is there anyone out there with decent metallurgical credentials who can confirm (or otherwise) whether there are issues with the approach that I’ve outlined? I’ve convinced myself that it’s OK, but have I missed something?

 

Oh - that's interesting. Heat and a scalpel is exactly how I'd have approached that job as well. Good to hear that you have it sorted without too much drama!

 

Okay - thanks for all that...

I should report - I've tried the Harpic Max Strength and it does work. It didn't get everything, but it's definitely worth a shot. I rinsed off very thoroughly and everything seems fine (6 months later).

I did try the oven cleaner bag thing on the headers on my Ducati - it didn't do anything at all. I figure that it's mainly supposed to be removing baked-on oily stuff, so perhaps it's not surprising that it didn't work.

Don't try the welding pickling acid - sounds like dangerous stuff.

I was a little worried about the acid in the harpic removing the passivating layer, so I did a bit of research. Seems that nitric acid is the stuff to use - but it's nasty to deal with. Apparently, citric acid is also good for passivating, so that might be worth a go - I suppose wrap a bandage round the header and soak it in a citric acid solution (citric is used for some preserving and also for descaling, so it's easy to get hold of). Dont' know for how long to leave it. I figure that the stainless must passivate anyway over time, but perhaps some help would stop the corrosion before it starts...

Any ideas out there?