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?
I'm interested in this too. The lovely headers and the exhaust mufflers on my Thruxton R yesterday became covered in molten plastic as a result of covering the bike with my nice (not any more) Oxford Products dust cover within a couple of minutes of ending a ride. I did it without thinking because I've been doing it since it was new 20 years ago with my Honda Hornet. The difference is that the Hornet's headers drop straight down the front of the engine and never come anywhere near the dust cover - and the exhaust muffler has a very effective heat shield. Sorting out the mess required: Running the engine for a few seconds to warm the molten plastic Carefully shaving off as much plastic as I could with a brand new Stanley knife blade between forefinger and thumb Letting the headers go back to ambient temperature Carefully rubbing the headers with some water and emery paper to retain the brushed finish For the exhaust mufflers I simply used Autosol paste and a new microfibre cloth
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!
I use Harpic Max to remove the browning of the stainless. I have seen no I’ll effects. I apply with a brush and leave for 5 minutes and then rinse thoroughly. Once dry I then autosol to restore a nearly new look finish.
I did this with my Daytona headers when I did the head gasket. I used the Harpic blue one and it did a great job of removing a lot of the staining, however I did need to use some Scotchbrite to get the stubborn bits off, then Autosol to polish up. I finished off with Belgom Alu which leaves a protective wax coating. No I’ll effects as yet.
This one is definitely not a good idea! Pickling acid is a post weld treatment for removing the heat scaling i.e. passivation layer and eliminating corrosion. It is based on HydroFlouric acid (HF) which if you look it up is a particularly nasty substance. Burns (chemical) can be of a very deep nature and this stuff can actually eat bone. The immediate first aid procedure is to dress the burn with Calcium Gluconate gel/paste in order to neutralise as much as possible. This should ALWAYS be available when the above paste is used. In this modern H+S environment chemical cleaning such as this has lost out to other safer, less liability inducing methods. Please don't consider this route for header cleaning!!!!
Whole heartedly agree with Eldon here, I’ve seen the damage HF can do. I used to work in the nuclear industry where it’s used extensively and once saw a guy get his arm covered in it. Despite immediate medical intervention he had to have his arm amputated because the HF just kept eating into it. Horrible stuff, stay well away!
If i remember right Freck and maybe you can substantiate it, at one time, not sure about now, insurance companies were very wary about selling burnt out vehicles. I'm sure I read somewhere that certain burnt O rings turned to goo and this had HF properties. Very nasty horrible stuff!
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?
