zinc coating and microbiological corrosion

[Mike Adams]

I am going to have a boat shot blasted and hot zinc sprayed followed by an epoxy coating. There seems to be a lot of talk about this microbiological corrosion and I wanted to ask what anyone thought about the properties of the zinc coating in preventing this type of corrosion or if anyone had any experience in this area. Clearly this is an expensive process and I know from previous experience how good it is in stopping general corrosion the question is does it work for microbiological as well or does it just carry on under the zinc surface?

[Alan de Enfield]

1 minute ago, Mike Adams said:

I am going to have a boat shot blasted and hot zinc sprayed followed by an epoxy coating. There seems to be a lot of talk about this microbiological corrosion and I wanted to ask what anyone thought about the properties of the zinc coating in preventing this type of corrosion or if anyone had any experience in this area. Clearly this is an expensive process and I know from previous experience how good it is in stopping general corrosion the question is does it work for microbiological as well or does it just carry on under the zinc surface?

 

Interesting question.

As zinc coating is simply painting by another name (its not like galvanising) It may allow Microbial corrosion to survive and continue to spread.

 

If there is any suggestion that microbial action is already in situ then it should be washed out before applying the zinc paint.

Maybe the blasting back to bare steel is sufficient.

 

 

If a hull is found with evidence of microbial attack, it is necessary to deal with it to try to prevent it recurring.

A simple solution is for the whole area to be washed with copious amounts of high pressure fresh water. When dry the area affected should be coated with a strong bleaching agent (sodium hypochlorite) diluted 1:4 with water and left for twenty four hours.

 

Afterwards a second high pressure fresh water wash is necessary followed by recoating. This will probably remove around 90% of the microbes but the only real solution is to blast back to bare steel and to treat any inaccessible areas such as tack-welded rubbing strakes as best one can with the bleach solution before applying the next stage of the coating process.

 

The main problem is that the microbes can continue to live beneath the existing paint coatings and once sealed in with a fresh blacking, the lack of oxygen and light is the perfect environment for them to thrive leading to a risk of corrosion from the inside out.

 

No coatings are entirely proof against a microbial attack from the exterior. Minute pinpricks, mechanical damage below the waterline are all opportunities for the microbes to penetrate the steel and commence the process from the outside in..

 

WARNING SODIUM HYPERCHLORITE IS HIGHLY CAUSTIC AND TOXIC. IT MUST BE TREATED WITH GREAT CARE AND RUBBER GLOVES, WELLINGTON BOOTS AND EYE SHIELDS ARE ESSENTIAL. FINAL FINISHING

If the pressure washing has exposed areas of bare steel, it is recommended that a zincphosphate rust prevention system such as Fertan be applied. This should be allowed to work over a 24 hour period and MUST be thoroughly washed off with water and a brush to ensure that only the bare steel retains the Fertan before a top coat of Keelblack is applied. This is essential to ensure that any subsequent coating is properly attached to the hull. Nevertheless, the microbes can still live underneath adjacent prior paint coatings so the only certain way to remove the risk of future attacks is by blasting back to bare steel – an expense many owners may not wish to contemplate.

[LadyG]

I believe my boat was so treated from new, it's now seventeen years old, and the original coating looked great two years ago when on the hard. I don't know if it has ever been re-epoxied, but there is no corrosion of any kind.

So, have the job done properly and sleep soundly.

Edited April 1, 2021 by LadyG

[ditchcrawler]

Didn't the surveyor pass comment on the hull coating/condition 

[Phil.]

1 hour ago, Mike Adams said:

I am going to have a boat shot blasted and hot zinc sprayed followed by an epoxy coating. There seems to be a lot of talk about this microbiological corrosion and I wanted to ask what anyone thought about the properties of the zinc coating in preventing this type of corrosion or if anyone had any experience in this area. Clearly this is an expensive process and I know from previous experience how good it is in stopping general corrosion the question is does it work for microbiological as well or does it just carry on under the zinc surface?

I can’t answer your specific point on microbial corrosion, but we had our hull so treated about six years ago, and at and below the waterline, it is still sound with no rust at all.

[BEngo]

The process of blasting and then spraying with molten zinc is pretty certain to terminate any corrosive bacteria.  The Keelblack puff Alan has posted suggests that blasting alone will do this.

Once blasted and sprayed, the zinc  coating alone will not have much if any bactericidal effect.  The key therefore is to get a good bond between the epoxy and the zinc and the zinc and the steel.  This will ensure that there is no crevice between epoxy, zinc and steel for the bacteria to settle in.  Getting those bonds is down to application techniques, but that is true for general corrosion as well.  If properly applied the boat will be well protected when it goes back in the water.

 

The characteristics of epoxy paint are such that if mechanical damage penetrates the paint there is not usually damage to the bond away from the immediate damage.  This means the damage does not form a home for bacteria.  Normal corrosion is going to happen to the damaged bits, but Mg anodes will deal with that, as will the zinc for small areas.

 

As with others above, we had Jarrah blasted and epoxied sides and base in 2010 and, apart from the odd scrape, the coating is lasting well with no corrosion apparent. The coating was checked in 2015 and 2019 and wire brushed and overcoated after sorting out the damaged bits.

N