Pinholes in epoxy coating submerged in potable water

[yahoo123]

I remember reading that pinholes in the epoxy coating is all it takes to seriously corrode the underlying material and that the epoxy coating is basically useless when the coating is scratched up.

I'm thinking this might be true for galvanic corrosion between two dissimilar metals in contact. But what about in submerged applications? Will a small pinhole in the coating of a submerged steel plate be the cause of a serious corrosion failure? I thought it was the wetted area that matters.

 

[ash9144]

Pretty much everything you say is correct. Pinholes or tears in a liner basically render them useless and depending on corrosion mechanism a liner with holes makes the situation worse. The pinhole allows for that small area to become a wetted area and that can set up galvanic corrosion between the wetted and non wetted area accelarating corrosion.

 

[brimmer]

Yes, when you have a pinhole the underlying metal comes into contact with the electrolyte (or becomes wetted)and the corrosion cell can begin... Usually once started, the pinholes will get larger and larger. Also remember when you have these small coating fualts, you have the large cathode (coated pipe) small anode (metal at pinhole) scenario which speeds up the corrosion rate, so a very small coating fault can cause severe localized pitting.

 

[yahoo123]

Thanks for the replies.

I thought when you have a small anode and large cathode, it should be fine. For example, using stainless steel bolts in a galvanized steel frame is not much of a problem, but if you're using galvanized steel bolts in a SS assembly, you will have a huge problem.

Also, how would carbon steel fare when continuously submerged in potable water without any coating?

 

[brimmer]

No, the cathode is a main driver of the corrosion rate. What you have described above makes perfect sense, perhaps you were just a little mixed up on what is the anode/cathode. The anode is the more electronegative metal (galvanic steel approx -1v), and the stainless approx -0.15v is more electropositive making it the cathode. So, in scenario A (stainless bolts and galvanized frame) you have a small cathode (stainless bolts) and large anode (galvanized frame) so there will be minimal corrosion. In scenario B (galvanized bolts and stainless frame) you have a large cathode (stainless frame) and a small anode (galvanized bolts) and thus the bolts will corrode.

 

[EdStainless]

Most coated wet systems use protection by sacrificial anodes. These anodes will see very low consumption rates since there is little or no exposed metal that they need to protect. If there are pin holes or damage to the coating the anode would be consumed more rapidly as it provides protection to the exposed metal. In industrial systems there is usually a mechanism to measure the current flowing through the anodes. My testing this periodical you can tell if you have suffered coating damage.

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Plymouth Tube

 

[unclesyd]

If not coated or if coated and the coating is damaged you always have opportunity for the "Bugs of Rust", MIC to establish themselves.
We always used a coating that could be repaired if there were breaches in the coating. We checked our water tanks with a "bird dog" prior to commissioning and during periodic inspections.

Anything is better than the coating that was used when I started in industry. The coating was pure Lanolin applied by hand. Lots of fun in elevated water tanks. All the girls were envious of the applicator's hands from working with Lanolin all day,smooth as baby's butt.

 

[strider6]

if you have a surface of steel exposed to water (potable or sea water) it will corrode. It's not necessary to have a galvanic coupling to start corrosion. One of the scope to apply a cathodic protection is to protect the surface coated where there could be some damage of the coating.
Corrosion rate for carbon steel in potable water is a function of several factors like: velocity of the water, temperature, oxygen content, carbonate content... usually the velocity of corrosion in potable water is not constant but decrease with time due to formation of protective scale.
a corrosion rate of 0.1-0.3 mm/y can be a realistic value..

S.

Corrosion Prevention & Corrosion Control

 

[fdac]

Concurring with others; CP is the usual way to protect any exposed steel at coating defects e.g. inside reservoirs. Almost no coating is perfectly pinhole-free; CP is a fairly low-cost addition to the corrosion protection.