stainless steel

We took the dinette off the SS hand hold/roll bar/benson in the main cabin: silly me hanging a cloth with 3M metal polish on it that I had used to clean all the rest of the stanless with. The cloth hung there for a week, now the SS finish looks dark grey

Any way of restoring it?

 
I would leave them unpainted. This will make regular inspection easy. The paint will only hold any moisture against the bolts which may start crevice corrosion etc.

Sure you could coat them in grease etc but it won't last.

Leave them uncoated and just check them every year by removing one nut at a time and checking for pitting etc.

Scotty

 
You passivate SS with with an acid wash,, typically something like Nitric... be careful.. You can really mess it up if you don't clean it really well first.

Passivating Baths

After thorough cleaning, the stainless steel part is ready for immersion in a passivating acid bath. Any one of three approaches can be used—nitric acid passivation, nitric acid with sodium dichromate passivation and citric acid passivation. Which approach to use depends on the grade of stainless steel and prescribed acceptance criteria.

More resistant chromium-nickel grades can be passivated in a 20 percent-by-volume nitric acid bath (Figure 2). As indicated in the same table, less resistant stainless grades can be passivated by adding sodium dichromate to the nitric acid bath to make the solution more oxidizing and capable of forming a passive film on the surface. Another option, used in place of nitric acid plus sodium dichromate, is to increase the concentration of nitric acid to 50 percent-by-volume. The sodium dichromate addition and the higher nitric acid concentration both reduce the chance of undesirable flash attack.

The procedure for passivating free-machining stainless steels (also shown in Figure 2) is somewhat different from that used with the non-free-machining stainless grades. That is because the sulfides of sulfur-containing free-machining grades are partially or totally removed during passivation in a typical nitric acid bath, creating microscopic discontinuities in the surface of the machined part. Fig. 2—Procedures for passivating stainless steel parts in nitric acid baths are fairly straightforward.

Nitric Acid Passivation of Stainless Steels


—Chromium-Nickel Grades (300 Series)

—Grades with 17% Chromium or more (except 440 Series) 20% by vol. nitric acid at 120/140°F (49/60°C) for 30 minutes —Straight Chromium Grades

(12-14% Chromium)

—High Carbon-High Chromium Grads (440 Series)

—Precipitation Hardening Stainless 20% by vol. nitric acid + 3 oz. per gallon (22 g/liter) sodium dichromate at 120/140°F (49/60°C) for 30 minutes

or


50% by vol. nitric acid at 120/140F (49/60°C) for 30 min.

Passivation for Free-Machining Stainless Steels Including AISI Types 420F, 430F, 440F, 203, 182-FM

and Carpenter Project 70+® Types 303 and 416


1.5% by wt. sodium hydroxide at 160/180°F (71/82°C) for 30 minutes.

2.Water rinse.

3.20% by vol. nitric acid + 3 oz. per gal. (22 g/liter) sodium dichromate at 120/140°F (49/60°C) for 30 minutes.

4.Water rinse.

5.5% by wt. sodium hydroxide at 160/180°F (71/82°C) for 30 minutes.

6.Water rinse.

Even normally efficient water rinses can leave residual acid trapped in these discontinuities after passivation. This acid can then attack the surface of the part unless it is neutralized or removed.

 
You passivate SS with with an acid wash,, typically something like Nitric... be careful.. You can really mess it up if you don't clean it really well first.

Passivating Baths

After thorough cleaning, the stainless steel part is ready for immersion in a passivating acid bath. Any one of three approaches can be used—nitric acid passivation, nitric acid with sodium dichromate passivation and citric acid passivation. Which approach to use depends on the grade of stainless steel and prescribed acceptance criteria.

More resistant chromium-nickel grades can be passivated in a 20 percent-by-volume nitric acid bath (Figure 2). As indicated in the same table, less resistant stainless grades can be passivated by adding sodium dichromate to the nitric acid bath to make the solution more oxidizing and capable of forming a passive film on the surface. Another option, used in place of nitric acid plus sodium dichromate, is to increase the concentration of nitric acid to 50 percent-by-volume. The sodium dichromate addition and the higher nitric acid concentration both reduce the chance of undesirable flash attack.

The procedure for passivating free-machining stainless steels (also shown in Figure 2) is somewhat different from that used with the non-free-machining stainless grades. That is because the sulfides of sulfur-containing free-machining grades are partially or totally removed during passivation in a typical nitric acid bath, creating microscopic discontinuities in the surface of the machined part. Fig. 2—Procedures for passivating stainless steel parts in nitric acid baths are fairly straightforward.

Nitric Acid Passivation of Stainless Steels



—Chromium-Nickel Grades (300 Series)

—Grades with 17% Chromium or more (except 440 Series) 20% by vol. nitric acid at 120/140°F (49/60°C) for 30 minutes —Straight Chromium Grades

(12-14% Chromium)

—High Carbon-High Chromium Grads (440 Series)

—Precipitation Hardening Stainless 20% by vol. nitric acid + 3 oz. per gallon (22 g/liter) sodium dichromate at 120/140°F (49/60°C) for 30 minutes

or



50% by vol. nitric acid at 120/140F (49/60°C) for 30 min.

Passivation for Free-Machining Stainless Steels Including AISI Types 420F, 430F, 440F, 203, 182-FM

and Carpenter Project 70+® Types 303 and 416



1.5% by wt. sodium hydroxide at 160/180°F (71/82°C) for 30 minutes.

2.Water rinse.

3.20% by vol. nitric acid + 3 oz. per gal. (22 g/liter) sodium dichromate at 120/140°F (49/60°C) for 30 minutes.

4.Water rinse.

5.5% by wt. sodium hydroxide at 160/180°F (71/82°C) for 30 minutes.

6.Water rinse.

Even normally efficient water rinses can leave residual acid trapped in these discontinuities after passivation. This acid can then attack the surface of the part unless it is neutralized or removed.

BTW, if you try to purchase Nitric acid the ATF folk might be interested in you.

 

Ishmael

52,541
12,323
Fuctifino
I would leave them unpainted. This will make regular inspection easy. The paint will only hold any moisture against the bolts which may start crevice corrosion etc.

Sure you could coat them in grease etc but it won't last.

Leave them uncoated and just check them every year by removing one nut at a time and checking for pitting etc.

Scotty
Sorry, wrong thread. Two doors down, to the right.

 

Point Break

Super Anarchist
26,324
3,985
Long Beach, California
I would leave them unpainted. This will make regular inspection easy. The paint will only hold any moisture against the bolts which may start crevice corrosion etc.

Sure you could coat them in grease etc but it won't last.

Leave them uncoated and just check them every year by removing one nut at a time and checking for pitting etc.

Scotty
Sorry, wrong thread. Two doors down, to the right.
Still, I got it. Thanks

 

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