Is painting over flash rust ok? 3

[Swedishrigpig]

On our ship we use a company that uses hydroblast (UHP) Ultra Hight Pressure guns.
The result if fantastic, very fast and effective. However after just a few hours or less, the metal clean surface goes light brown, I believe it is called "flash rust" and the company says it is perfectly fine to paint over it. This goes against everything I ever learned about corrosion protection and painting.

Is he correct, or will I see the paint peel off in six months time?

 

[unclesyd]

Check with your paint manufacturer as some can and some can not be used over flash rust.

There are products that can be sprayed on the clean surface that will prevent the rust and can be painted over.

 

[gr2vessels]

As correctly said by unclesyd, sometimes (when specified in your painting specification) you can apply on top of the cleaned surface the primer which can be a rust inhibitive paint, which will neutralise / change the type of ferrous oxide to a stabile form. Then you apply the mid coat and the top coat, in accordance with your paint spec.
However, in many cases, the flash rust is not acceptable and that must be clearly stated in your specification, so the cleaning contractor will co-ordinate his work with the painter, who will apply the primer coat within the time specified by the cleaning contractor.
If you don't have that detailed specification, you must make sure the primer is a rust inhibitive type and that it will be applied rather sooner than later after the blast cleaning.
Cheers,
gr2vessels

 

[SJones]

First check the datasheets and associated product literature for the paint system. They should state the required minimum surface preparation grade and the visual requirements for substrate cleanliness and any allowable flash rusting in accordance with ISO 8501-4/NACE No 5. If you don't see that and have to approach the paint supplier, ensure that they are able to provide a documented test report of the paint performance after application to a flash rusted surface to back up what they say.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[btrueblood]

Can you not add a rust inhibitor (TSP is a suggestion) to the hydro-blast water, or add a step to spray such an inhibitor on the blasted surfaces before they dry?

 

[Swedishrigpig]

Good info there gents.
I will check our paint specs. We use Hemple marine paints:
I will attach the maunals for you to see.

About the rust inhibitor, please tell me more, we use Reverse Osmosis water onboard and that is very aggresive which doesnt help.

 

[Swedishrigpig]

Sorry, missed the first page.

 

[hokie66]

The Hempel specification requires a Sa 2 1/2 surface. Leaving the surface to flash rust is not in accordance with the specs. The epoxy zinc primer performance would definitely be diminished by the rust, but by how much is anyone's guess. I would get the manufacturer involved.

 

[btrueblood]

Swede, sorry not to get back to you sooner. Talk to these folks, or google some of the terms used on their site.

http://www.deforest.net/corrosion.html

We use one of their products (DePhos 8028) in an immersion tank for leak testing, but we are looking for corrosion inhibition of multiple metal types (copper-based, zinc, steel).

You might try using a rinse that has been treated with enough trisodium phosphate (TSP) to raise pH above 9 or so. We did this when washing some steel parts that needed to be degreased prior to painting, adding a TSP rinse helped to prevent (or at least drastically reduce) flash rust as the parts dried. The solution could be considered hazardous, but if you are spraying high pressure water, you should already be collecting the waste stream, and wearing protective gear. TSP may leave a residue, but we find it wipes off easily with a dry rag prior to painting.

There are also more involved phosphating treatments for steel which provide better, longer-term corrosion protection, better paint adhesion, etc. etc. yadayada. You can google for those also.

 

[unclesyd]

Here is a Hold Tight that I use on some boats and it is also sued by a large local coating company, both for paint and powder coating. I would contact them as they have a long list of approvals.

http://www.holdtight.com/index.html

 

[MrBTU]

Rust bloom under any zinc rich epoxy is a disaster. Other coatings are more forgiving. If our customers spec zinc rich epoxy, we steel shot blast immediately prior to coating.

 

[hokie66]

MrBTU, I believe that is overstating the situation. Coating over rust bloom is unacceptable, but zinc rich epoxy coatings are actually more forgiving than inorganic zinc coatings. That is why the epoxy primers can be used in field repair applications, while inorganic zinc cannot.

 

[HgTX]

Dunno...epoxy zinc primers are more forgiving of crappy application than IOZ, but that has more to do with mudcracking in IOZ if it gets too thick in the nooks and crannies, and the nonrecoatability of IOZ. The zinc dust itself still doesn't accomplish anything if it isn't in direct contact with the steel. If there's rust on the steel, you don't have contact.

Hg

Eng-Tips policies: faq731-376

 

[hokie66]

HgTX,

I'm certainly not advocating applying these primers over rusted surfaces, but the epoxy part of the zinc filled epoxy primer does provide a degree of barrier protection, while the IOZ does not. In situations where poor surface preparation is inevitable, a barrier type primer (e.g. Carbomastic 15) should be used.