Need help about polishing stainless plate

[By Invitation]

Hi there,

I have just had 14 chain plates fabricated from 316. I need to know from you experts just how polished the surface needs to be and what treatments if any should I do before I re-install them. These were made from regular 316 flat stock and were pretty dull when I got them. I have polished the entire plate with a high speed 120 grit grinder to remove all the small pits and machining marks. I have polished the top (exposed section when installed on the boat) ends to a mirror finish. You can still see very small marks from the grinder if you look close. Do I need to polish any further for any other reason than cosmetics? Do I need to polish the lower half for any other reason than cosmetics? I there anything I need to do before I install them?

Thanks

Invitation

[Alan Wheeler]

Nup, the polishing is for your interest in looks only. If you are happy, then that is all that is needed.

[GordMay]

The stainless steel properties are obtained by a unique composition of the alloy in which chromium plays the leading role. Chromium combines with oxygen to produce an extremely fine impenetrable film of chromium oxide, which protects the underlying material. When this film of chromium oxide is present the stainless steel is corrosion resistant in which case we may call the metal passive.

It is during construction (cutting & grinding) and handling that this layer may become damaged, by atmospheric contaminants, iron, paint and grease. In those places in which the oxide layer is damaged and other forms of contamination prevent the passive film from reforming naturally, then corrosion can take place.

‘Pickling’ and/or ‘Passivating’ will restore the corrosion resistance. This is achieved by stripping the contaminated oxide layer, via etching, and allowing a new layer to form under controlled conditions

Most passivation solutions have an acid base (Citric, Nitric, or Anhydrous Hydrofluoric) which cleans (dissolves any free iron or other contaminants from the surface) the metal, and re-oxidizes (the chromium) and instantaneously restores the passive layer to the Stainless. Industrial Passivation of stainless steel is normally accomplished by dipping the part in such an acid bath. These are all very dangerous chemicals !!!
- Clean/degrease in 5% Sodium Hydroxide (NaOH) or Oxalic Acid (HO2CCO2H) at 160 to 180̊F (71 to 82̊C) for 30 minutes. (some drain cleaners).
- Water rinse.
- Passivate in 20% Nitric Acid (HNO3) or Citric Acid (H3C6H5O7 ) for 30 minutes at 120/140̊F (sometimes Sodium DiChromate is added)
- Water rinse.
- Dry & Install.
If you are only planing to passivate a few parts, you are better off taking them to a plater, or metal finisher.

DIY
But you don't need a nitric acid bath to passivate. The key is to clean the stainless steel to bare metal. The best way for the DIY to do this, is to use an oxalic acid based cleanser and a non-metallic green scrubby pad. Once the metal is clean (and dry), the oxygen in the atmosphere will form the protective chromium oxides. The steel will be every bit as passivated as that which was dipped in acid. The catch is that it takes longer - about a week or two.

HTH,
Gord May

[By Invitation]

GordMay & Alan,

Thanks to both of you for your respones. I'm getting closer here.

Thanks

Scott
S/V Invitation

[Alan Wheeler]

Thanks Gord, very informative once again. However, Pickling is usually only done if the SST has been welded or heated. A Pickling paste is available by the way and should be obtainable from a SST supplier or engineering supply store.
Usually a plain piece of SST will not require this. SST oxidises extrememly rapidly. If the SST has been well polished to removes scratches and burrs, then that is usually all that is needed. If it has been heated and/or welded, then the surface layer is damaged by "other" oxides coming to the surface. This is what causes the Blueish color. This layer will corrode. Plus the imediate edge of a weld is another. The chemical structure of the metals is just slightly different along that fine line and corrosion often sets in along that. It's why you will often see rust along a welded portion of SST. As you stated, the acid restores that chemical balance to those area's. The acid eats away those contaminants and "cleans" the chromium to enable it to oxide in the way it was supposed to, allowing that barrier to once again protect the surface of the steel.

[Rick]

Where can I obtain citric acid (other than squeezing an orange or lime)? I've used nitric acid to clean the oxide from welds yet do not like to have that nasty stuff around.

I know that with citric acid and a power supply I can possibly get a more controlled reduction.

Thanks,
Rick

[CSY Man]

I had my 316 SS chain plates electro polished at a chrome shop.

Fairly in-expensive and no hassle.

[mazagran]

very usefull and cheap is a very fine white powder sold in German speaking countries as "Wiener Kalk", found in most drugstores. Actually it is very fine ground Dolomite Powder CaMg(CO3)2.

Maybe someone knows a more special english tradename.

Whenever there are dirtmarks on metal, just wipe it off. We usually keep it in a metal jar, the cloth used put inside, never changed, surprisingly, the dirtier it looks, the better it cleans.

If you like a superb polishing paste use 1 part water, 1 part ammonia solution, 2 parts isopropyl alcohol and 1 cup powder. Adjust properties by adding more or less water or powder

Cleans & polishes every metal and hard paint without making scratches! Do not know anything better for cleaning stainless

[Rick]

Thanks for the great tip!
Rick

[Richhh]

Polishing is important to reduce the tendancy of the formarion of fatigue cracks. Mechanical Polishing to a 'mirror' surface is the goal. .... then electropolish if available to really enhance the corrosion and fatigue resistance.

Sanding is the fastest way -- 100 grit followed by 180 grit, then 200, 340 then 400. Then you power buff with specialty buffing compounds such a "tripoli" which is a naturally mined mixture of dolomite clay and diatomaceous earth. ,,,, then if available, electropolish. You can buy 'tripoli' in most hardware stores. The electro-polishing will slightly 'dull' the surface but will remove most of the microscopic surface roughness and make the stainless more corrosion and fatigue resistant. If you cant find a source of electropolishing, mechanical buffing to a mirror surface is probably 'good enough'.

[By Invitation]

Thanks everyone,

I have mastered the art of polishing these things. Have a belt sander with a mounting bracket that holds it upside down securely. I use 120 grit on the belt then wet sand with 400 with an orbital then polish on a wheel. The result is a mirror finish in about 20 mintues for each chain plate. When I am done with all 14 I will pickle them and then install them this fall.

[StoutWench]

Both Mothers Mag Polish and Flitz Metal Polish work great of stanchions. How well will they help protect new chain plates that have not been elctro polished?

[Starbuck]

This source may prove helpful.

[Steve Kidson]

I have used a couple of different methods to re-junenate dull or badly scratched stainless;
1. I have a mate who is a sheetmetal worker, specialising in SS work, and I either have him come and do it for me, or borrow his polisher
2. Scotchbrite pad and WD-40 or RP-7, rub with the grain
3. Gumption and a wettex cloth - gumption is a fine paste used for cleaning and polishing almost anything.

It is important to remember that for the best result, polish or rub with the grain, if the linished look is what you are after.

Steve