Question about pickling carbon steel

[Cheetos]

Hi, I have a question about the machined steel plate I received from overseas vendor. The entire plate has a medium grayish finish. I tried to scrape it with a screw driver, and I don't see anything flake off, so I don't think it is a primer. I have 2 questions:

1) After you machined a plate, the plate should have a shining surface. If you pickle the plate afterwards, will it create a grayish finish?
2) I read that pickling is a rust removal technique, not a rust preventive measure, so after you pickle the carbon steel, do you still need to put rust preventive coating, like oil, to prevent it from rusting during the shipment?

 

[TugboatEng]

Yes a coating is required to prevent rusting during shipping. Parts that cannot be coated are packed in sealed packaging with vapor phase corrosion inhibitors.

Cast irons have a gray surface finish after machining. Have you tried sanding it to see if it brightens?

You could also try applying an acid to the surface to see if it brightens or corrodes.

 

[EdStainless]

Typically a freshly machined surface would be shiny, and after acid pickle it would have a slightly matt or dull finish.
You would need to apply a Rust Preventative oil or similar coating right after pickle to prevent rusting.
There are many RP oils available, different base oils and different weights depending on the application.

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P.E. Metallurgy, consulting work welcomed

 

[Cheetos]

This got me thinking. If pickling is a rust removal technique, why pickle + rust preventive oil vs. rust preventive oil?

 

[TugboatEng]

RPO doesn't remove rust that has already formed. For example, if the part wasn't properly dried after machining with a water based coolant it may have started to corrode prior to the RPO being applied.

 

[EdStainless]

The pickling will also remove any smeared metal or small flakes/burrs.
It will be a clean surface.
Acid, rinse (usually 2) (hot clean water, not tap water), hot air blast to finish drying, dip in RP.

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P.E. Metallurgy, consulting work welcomed