What corrosion process is going on in this steel deck?

[boden36]

Thanks Stillbuilding. If you blast with a grit like garnet, you should have no oxide to convert. Putting rust converter on an un rusted area is not recommended.
I would not worry about grinding around the pitted areas, as you are just taking away good steel and not the rust in the pit. A needle gun will take most of the corrosion out of the pit, and you should not have to abrasive blast much.

Regards,
Richard.

[stillbuilding]

Originally Posted by boden36

Thanks Stillbuilding. If you blast with a grit like garnet, you should have no oxide to convert. Putting rust converter on an un rusted area is not recommended.
I would not worry about grinding around the pitted areas, as you are just taking away good steel and not the rust in the pit. A needle gun will take most of the corrosion out of the pit, and you should not have to abrasive blast much.

Regards,
Richard.

Thanks for the hope Richard.

I am assuming the worst and will have enough phosphoric acid available when blasting but ideally will not need.[emoji120][emoji120]

I am indeed using garnet - pure coincidence - came as a package with the blaster.

Will update after blasting - new learning experience for me.

[stillbuilding]

Thanks to all the responders to the thread - nice to have the background knowledge when confronting a new situation.

[Aloha_float]

From a metallurgy standpoint - the chlorides under a permeable coating allows the establishment of anodic corrosion sites. Water permeating through the coating forms and electrolytic condition depolarizing the location and initiates a pitting corrosion condition. Clor-rid is good advice. A zinc epoxy may be a consideration however, I would double check with the marine experts. The zinc epoxy obviously provides the sacrificial anode to offset any potential depolarizing of the base metal. Then an intermediate epoxy with a topcoat designed for the marine environment. A two part polyurethane is the least permeable of coatings when it is compared to two part epoxies. It's been too many years since I have been in the coating industry that I have forgotten a lot of this detail.

I have seen some successful applications of moisture cured urethane with an aluminum primer however, proper material prep is also required.

[slowpoker]

Yes, make sure to scrub and wash off any of the salts remaining on the metal surfaces even after blasting and chipping, before applying any coatings. Soap and water may not be enough salt removal. Use the chemicals recommended here by Aloha_float and others.

[stillbuilding]

Originally Posted by Aloha_float

From a metallurgy standpoint - the chlorides under a permeable coating allows the establishment of anodic corrosion sites. Water permeating through the coating forms and electrolytic condition depolarizing the location and initiates a pitting corrosion condition. Clor-rid is good advice. A zinc epoxy may be a consideration however, I would double check with the marine experts. The zinc epoxy obviously provides the sacrificial anode to offset any potential depolarizing of the base metal. Then an intermediate epoxy with a topcoat designed for the marine environment. A two part polyurethane is the least permeable of coatings when it is compared to two part epoxies. It's been too many years since I have been in the coating industry that I have forgotten a lot of this detail.

I have seen some successful applications of moisture cured urethane with an aluminum primer however, proper material prep is also required.

Still not sand blasted but I have ordered in some Zinc Epoxy from Resene, a NZ company.

Drying time is 30 mins so not a big hardship to include post blasting.

So plan at this stage is sand blasting, spot painting of any uncleared rust lesions with Phosphoric acid, Zinc epoxy and the normal two part primer, Undercoat and top coat. And pray.

[stillbuilding]

A brief update for those helpful contributors.

Gave up on using a needle gun and grinding - just too much and too slow and ineffective.

Garnet sand blasting gave a 100% clean finish not needing Phosphoric acid wash. All the carbuncles cleaned out easily as you will see from the pic.



I will use a Zinc Epoxy base before Epoxy primer.

Update when done.

[boden36]

Looks good. Think about using a product like Chlor Rid
to clean the salts from the blasted steel prior to painting. Will be well worth the extra trouble.

Regards,
Richard

[valhalla360]

Way back I dealt with corten steel for guardrail. It was cheaper so we were told to use it...who cares if it looks rusty as long as it's solid.


We replaced a lot of guardrail within 5yrs with good old galvanized steel.


Just like stainless all steel can run into corrosion issues under the right conditions.


Complete encapsulation with no defects is critical. On a research project back in school we were looking at epoxy coating for rebar. Good quality coating was best. nearly as good was uncoated rebar which would just develop light surface rust but lost no significant structural capability. The worst was damaged or poorly applied epoxy. It actually had the affect of accelerating the rusting process.

[stillbuilding]

Originally Posted by valhalla360

Way back I dealt with corten steel for guardrail. It was cheaper so we were told to use it...who cares if it looks rusty as long as it's solid.


We replaced a lot of guardrail within 5yrs with good old galvanized steel.


Just like stainless all steel can run into corrosion issues under the right conditions.


Complete encapsulation with no defects is critical. On a research project back in school we were looking at epoxy coating for rebar. Good quality coating was best. nearly as good was uncoated rebar which would just develop light surface rust but lost no significant structural capability. The worst was damaged or poorly applied epoxy. It actually had the affect of accelerating the rusting process.

Valhalla,
That seems to have been the situation here - much more aggressive rusting with limited exposure to air. Surely had lots of rust on exposed areas but not so deep and none of the "carbuncle" lesions as found under the paint.

Blasting takes it all off but is slow process.

Just FYI, a needle gun was really slow going and only suitable for small areas. Hitting the plates with a hammer was more effective. I did not try a chisel point in the gun.

There is some Corten in the hull and this is much more rust resistant than mild steel INSIDE the hull - did not seem to stop underwater corrosion which has already been repaired by grinding/welding and replacing severely affected plates.

Happy days!!
Richard