Metalizing a carbon steel tank

[eieio]

The following procedures for coating the outside of two CS tanks were recommended by a NACE certified paint specialist on a project:

1. Tanks shop primed at the factory for shipping.
2. Tanks will be sand blasted / blast cleaned to NACE 1 / SSPC 5 (2 mils)
3. Tanks will be metalized with 100% Zinc (8-10 mils penetration / coating)
4. Tanks will be coated with a water-based, ultra white, high gloss white acrylic paint (8-12 mils)

I am being told by the design engineer that it is not possible to metalize with 100% zinc. He is recommending the following coating:

1. Tanks will be sand blasted / blast cleaned to SSPC 6
2. Tanks will be primed at factory for shipping
3. Tanks will be coated with an epoxy intermediate coat and a polyurethane topcoat.

Additional recommendations for the following have also been sent: Tnemec 90-97 zinc system is the prime coat at 4 mils and the Pot-a-Pox 20 is two coats.

Can anyone lend some insight into this situation?

The tanks will be installed outside in a Caribbean location. There is little direct salt spray, but they will be within sight of a coastline.

 

[Unotec]

I've never heard of Zn 100%,in our tanks (internal) we use Zn/Al 85/15 and incanol on some impellers.

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[unclesyd]

We used pure zinc thermal spray on several waste tanks as a primer coating as proposed by your consultant. We also used pure zinc along with pure Aluminum in several other applications especially some big pipe. I don't recall our topcoats at the time.

Just saw some thermal spraying concrete with pure zinc.

I thought there might be some environmental restrictions on pure zinc but have found none.

 

[SJones]

eieio - are you the end user or the supplier of the tanks? Your view on what is happening will be driven by your role in the process. Both options are feasible - it's just that one is probably a lot cheaper than the other when up front capital expenditure is the key. Sadly, the cheaper capital expenditure may lead to increased operating expenditure when it comes to having to maintain the paint system. Try to get clarity on why it is not possible to thermally spray the tanks at site. It may be that there are no qualified thermal spray contractors readily available. It is certainly not because the consumables are not available. If you are the end user try to work out whether your company is happy to have to fully repaint say every 10 years versus only having to touch up the top coat to maintain aesthetic pleasantries for probably the design life of the tanks.

Steve Jones
Materials & Corrosion Engineer

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

 

[eieio]

I am indirectly the end user of the tanks as the Proj Mgr/Facilities Eng.

We do have a NACE certified guy to do the metalizing at the site, but politics may prevent him from being able to do it. And there is no guarantee he will stay forever, either.

Otherwise, the contractor installing the tanks would have to bring someone & equipment to the site to do the job.

Routine maintenace is not a problem, as long as it gets done. Interestingly enough, the routine maintenance would likely be done by the guy who may not be allowed to coat the tanks in the first place. (Politics again - different color of money)

Given this information, which system seems to be the best fit?

 

[SJones]

OK lets start with the key to the matter: surface preparation. Coating performance and durability will be dictated by the quality of surface preparation. Look at the two options - which has the best specified surface preparation? Then let's look at coating degradation: firstly, with the epoxy system, you are going to rely on the shop primer being in a good state by the time it comes to overcoat with the epoxy. Possible but not likely. Secondly, unless the primer is a zinc rich, you will not get any benefit of cathodic protection on the carbon steel. Having said that, it has just jogged my memory that zinc may suffer an elevated temperature potential reversal and, being in the Caribbean sunlight, you may wish to question your NACE man on whether thermal spray aluminium may not have been a better choice. Anyway, if we assume that both coating systems are applied to the same (high) quality, you will probably be looking at having to make substantial maintenance painting effort on the epoxy/PU system after, say, 10 years whereas on the TS/acrylic system your expectation would be minor touch up and aesthetic improvement coating just to keep a decent white colour. You have to examine the cost and health and safety implications of having major coating works going on every 10 years versus a more or less fit and forget coating system albeit one that costs you more in the first place.

Steve Jones
Materials & Corrosion Engineer

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

 

[eieio]

Could we request that the tanks be metalized before shipping? Is this something that is done commonly or would it be a special request and therefore difficult/expensive?

 

[Unotec]

Should be less expensive than doing it in the field, I would think. At least here it is

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>