For about $4, you can buy a gal of muriatic acid (dilute) at any hardware or pool store which works far better than anything else suggested, doesn't damage anything and is very easy to use.
I've used it on SS, aluminum, gelcoat stains and starboard stains equally well. Just rinse off when you are done and wear latex or neoprene gloves.
Muratic (hydrochloric acid) should not be used on stainless. It dissolves the passive chromium oxide layer that is integral to stainless' corrosion resistance and will end up corroding the s/s. Search any "chemical resistance of stainless" chart and you will see there is severe effect with even low concentrations of HCL. In a "salty" marine environment this is even worse because the chlorides in the salty air will begin to attack the surface before the CrO layer has time to autopassivate. Your part will fail prematurely. If you must clean rust in-situ, phosphoric/citric (better for environment) or nitric (not good for environment) formulations. Do not use muratic/HCL, You have been warned. I will post pics shortly.
#2 shows comparison of original bolt vs muratic acid submerged bolt (black)
#3 close up of submerged bolt with smut wiped off. Note:size reduction and rough surface finish.
#4 comparison of original bolt vs muratic submerged bolt. The submerged bolt was also subsequently electropolished for a corrosion correction study of HCL effected s/s. Note:size reduction of HCL'd bolt this is as result of surface dissolution by HCL on s/s.
Dramatic pictures but there is no mention I noticed which describes the period of exposure. Are you implying these pictures reflect the process I described or are they the result of a prolonged "submerged" exposure?
Exposure was un-agitated 15 minutes followed by 12 hours un-agitated. The most noticeable effect was in the first 15 minutes. While it is unlikely that you would wipe or be able to contact an in-situ boat part for 15 minutes. Make no mistake the passive layer is being removed and your just rolling the dice for future corrosion and part failure.
That type exposure, as you describe, to produce those results is vastly different from simply and judiciously removing rust stains and flushing the residue with water but I can certainly agree that any prolonged exposure of ANY metal to acid can be detrimental.
Since I didn't even know SS had a "passive chromium oxide layer", think I'm going to listen to the Metalfinisher!
Spotless Stainless works wonders!! It is citric based so falls into the passivating/safe for SS category not the damaging/unsafe for SS category when it comes to acids. Nitric and Citric are the two acids that can physically passivate or repassivate SS. You can't easily get nitric.....
No other acids should be used on SS other than Citric or Nitric, unless you want to risk permanent damage.
Muriatic/Hydrochloric, Oxalic etc. should NOT be used on SS.. MetalFinisher is spot on about this.....
I tried to clean up o length of chain by soaking in diluted muriatic acid. It turned the galvanized chain black in about 2 minutes. I worked for an hour trying to get the black off using soap and hot water and a scrub brush. Finally throug the chain in the trash and got a new one.