Rust on new chain plates

[Diver22]

I purchased 316 alloy stainless steel flat bar and had a machine shop cut and drill it to match my existing chain plates on my Hunter 37 C. I asked him to polish the chain plates. He polished them, however they did not achieve a chrome like finish, the flat bar does not have an entirely smooth surface, it has liniar machining grooves in it. I left them on the pier last night in the rain, they now have spots of surface rust on them and rust where it was drilled and cut. Will the T9 rust inhibitor protect it from future rust? And what about the areas where it attaches to my shrouds? Will the T9 simply rub off and allow it to rust?

[transmitterdan]

Stainless steel needs a passivation step to activate its stainless properties.

However, the rust you see is probably residue from the machines used to sand and drill. They probably have non-stainless debris now deposited into the metal and that is what you see as rust. T9 won't stop it.

[Stumble]

First the shop didn't polish them, there should be no remaining tooling marks just the grain of the metal. Second, they used tools that were contaminated wi regular steel that has now cross contaminated the stainless.

The best option is to pull them again, and with a new polishing wheel start buffing. For the final step use a new polishing wheel (the old on is now contaminated too) and do the final polishing. Then passivated the metal in a citric acid bath for a few days.

[roverhi]

Electro polishing is a much better way to treat stainless. It doesn't give the chrome like finish that you can get with mechaincal polishing but does get into the nooks and crannies that mechanical polishing can't get at. If you want that super shiny finish, the SS can be mechanically polished after electro polishing. As othes have said, the rusting you see is most likely from embedded ferous particles from machining or contaminated polishing wheels. Passivating will mostly take care of it or just treating with rust remover and metal polish will keep the rust at bay until the metal particles are rusted away.

[GordMay]

Greetings and welcome aboard the CF, Diver.

[barnakiel]

The plates have not been properly passivized or else the bolts marked them or else the bolts are rusting.

Remove, polish then passivize. The electro method seems to deliver great results, imho.

b.

[Stumble]

Electropolishing also passivated the metal, and is the best option if it is available to you. But it isn't something you can do in a home shop you have to farm it out.

[wunderluster]

The OP said he left them on the pier in the rain. I'm pretty sure they weren't attached to the boat

[Cheechako]

Yep, get them passivated and electro polished at least. Buying 316 isn't a fail safe option for non rust. If you want them to look like chrome they will have to be mechanically polished, which can be superior.... but must be done properly with no steel contamination and by someone who knows what they are doing.

[Scaramanga F25]

Stain...less.

[coastalexplorer]

I think I read somewhere that it is the oxidization of the chromium that is alloyed in the steel that creates the protective chromium oxide coating to make it called stainless. Thus 'stainless steel' will rust IF you exclude adequate exposure of oxygen from it in the presence of water. or as stated in the other replies if it comes into contact with other ferrous metals that do not contain chrome.

[jhulmer]

Originally Posted by Stumble

First the shop didn't polish them, there should be no remaining tooling marks just the grain of the metal. Second, they used tools that were contaminated wi regular steel that has now cross contaminated the stainless.

The best option is to pull them again, and with a new polishing wheel start buffing. For the final step use a new polishing wheel (the old on is now contaminated too) and do the final polishing. Then passivated the metal in a citric acid bath for a few days.

I agree, this is my answer as well.

I own a machine shop and we do a lot of stainless work and we see this all the time.

carbon deposits from the tools used in the machining process. Either Mechanically polish, electro- polish (passivation), or chemically pickle (passivation) with something like this:
https://www.mcmaster.com/#8554t11/=16okg36

you can also check with a local welding supply for a weld pickling chemical which is used to remove carbide of welds on stainless.

then mechanically polish with Scotch brite wheel and then cotton wheel and rouge for a mirror shine.

you must get rid of it or it will continue to crevice corrode around the rust spots.

[buzzstar]

There are also minor but real possibilities that the supplier provided 304 rather than 316, or that the 316 was sub standard (did not meet actual specs). The prior responses that focused on questionable polishing and passivation are more likely to have identified the problem, but....

[PC45]

You have to use a stainless steel like Sandvik SMO 254 or Avesta Sheffield SMO 254 if you want real stainless
Crevice corrosion test: When stainless 316L corroded >3 mm (3 of 3 test panels) after 18 month submerged in the sea, watertemp. 5 - 30ºC, SMO 254 corroded 0,09 mm (only 2 of 12 test panels) in the same test.

Good luck!
Pelle

[Island Time O25]

What would be the total cost of getting this ss plate and cutting/drilling/passivating, etc? I recall in a brand specific forum a few years back (3-4 yrs may be) one owner posted that he scored a good deal on a titanium sheet for a few hundred $$ and had that cut and drilled into chain plates for an older 36 footer. I vaguely remember that his total outlay was around $400. I forget the size of his initial sheet but he had leftover piece that he sold cheaply to another forum member. Considering titanium's properties I gladly would pay 50% to double of the ss prices.