Pin Hole Corrosion Acceleration

[MechEng1977]

If I have a pin hole in a anti-corrosion paint, will corrosion be accelerated in the area where the pin hole occurs.
The paint is applied to a 1080 Rail Steel.

The Steel is currently corroding quite rapidly, and one of our ideas is to coat the steel with a anti-corrosion paint. One of the team members is concerned if there is a pin hole in the protection due to mis application or handling, the Rail will rapidly corrode in that area.

My thoughts are yes it will corrode in that area, but it won't spread and it is better than the entire rail base corroding as it is now.

Thanks in advance

 

[brimmer]

Corrosion would be accelerated in a pinhole area, as you have the scenario of a small anode, large cathode. Coating will probably get worse as corrosion depostis may build up in the area and spread under the coating from the inside causing the pinhole to get larger. Whether or not it would be better or worse to coat the entire rail base, probably better to have some rail base there over nothing, easier to repair one small section than replace the whole thing later. If coating, surface prep will be very very important, you must get the rail base clean before application of coating, otherwise you could make the corrosion worse on a large scale.

 

[SJones]

scenario of small anode, large cathode

If the coating is an insulator, where will the large cathode be?

Steve Jones
Materials & Corrosion Engineer

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

 

[MechEng1977]

This is what I don't understand: fact is, any coating be it Zinc, Phosphate, Epoxy has the potential for developing areas of 'pin-holes'. This means anything coated would be worst off than not coated at all?

If this were true, the coating industry would not exist and automotive car bodies would not be galvanized, bolts would not be phosphate coated, etc..

 

[brimmer]

cathode is the rest of the coated rail

 

[cloa]

MechEng1977
The zinc coatings have cathodic protection effects so a pinhole is protected by the zinc, the others are effective by increasing electrolytic resistance so the pinholes corrode at a normal rate.
The reference is to coatings that neither provide cathodic protection nor significant electrolytic resistance.

 

[unclesyd]

You might want to check the remainder of the coating with a bird dog especially around the existing pin hole to see if you have any additional electrical paths.

What is the operating enviroment the rail sees?

Do you know how thick, Dry Film Thickness, the coating is?

What was the surface prep prior to phosphating?

Did the part sit around after phosphating?

 

[Metalguy]

In your case there is no cathode with respect to pinholes. An intact paint/epoxy coating removes the rest of the rail electrically via insulation, as already stated.

Zinc coatings are well known for having good "throwing power", meaning they can protect bare steel that is some distance away from the zinc--say a few mm or more.

At room temps. Zn is anodic to steel and protects by slowly sacrificing itself. Tin, which is cathodic to steel, protects only where the coating is pinhole free

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[brimmer]

The pinholes are anodic to the rest of the rail, this is why the corrosion occurs... the current flow is through the electrolyte, the pinholes will get larger with time

 

[Metalguy]

The steel at the pinhole will corrode if/when wet, and said corrosion will probably migrate under the surrounding coating.

However "the rest of the rail" is NOT cathodic to the pinhole area, because the intact coating has removed it from the necessary electrical circuit. This is basic Corrosion 101.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[brimmer]

Well, at least now we are in agreement there will be corrosion at the pinholes (in the presence of electrolyte to state the obvious), and corosion will most likely spread outward at the pinholes which was the concern in the original question.

Yes, obviously there is no metalic path through the in-tact coating, the pinholes are still anodic to the rest of the coated surface

 

[brimmer]

Exerpt from text book to the original question with original concerns, which I still say there will be accelarated corrosion at pinholes and it will spread in future. In this case talking of epoxy coatings on anchor shafts for poles.

Galvanization and Epoxy Coatings
Epoxy coatings are also often used to protect shafts. This is accomplished by coating the entire shaft with an epoxy
that acts as an insulator to protect the steel shaft from direct exposure to the electrolyte. This method is beneficial in
protecting against corrosion, however, it has been proven that even the best epoxy coatings cannot guarantee 100%
isolation from current. In addition, the coating can be damaged during shipping or installation, leaving small
anomalies or "holidays." If the shaft is then buried with these "holidays," the "big cathode, small anode" scenario
spoken of earlier comes into play. The shaft is open to accelerated corrosion in small areas that rapidly become
larger. This type of concentrated deterioration is worse than if the shaft were left to corrode on its entire surface
more evenly.

 

[Metalguy]

Not to drag this on and on, but you cannot have a anode if there is no cathode. There will be tiny anodes and cathodes inside the pinhole area and they move around (electrically), but the entire pinhole area has no cathode, so it's not an anode.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[brimmer]

Final comment. Agreed. There will be many many small corrosion cells (anodes and cathodes) on the metal within the pinhole, generally speaking the outer edges of the pinhole will be cathodic to the smaller inner ring of the pinhole (I don't think I never meant to imply the entire metal under coating is the cathode as there is no metalic path to these areas, the metal at the edges of pinholes is - although reading above my comment "cathode is rest of coated pipe" when taken literally is incorrect (ooops)). The external current (in the electrolyte) is the driving force in this scenario, which is why corrosion will keep occuring.
If you were to measure electical potential of the pinholes that have been there, and a section of well coated pipe, there will be a difference in electrical potential, the pinholes will be anodic, rest of pipe cathodic. What I meant is (and still maintain) is pinholes are anodic in potential to the rest of the coated pipe is cathodic to pinholes).

Main thing here is I hope I demonstrated to MechEng 1977 there is merrit in the comments of the team member who was stating if there are pinholes, corrosion could occur rapidly in that area, and get worse. If not, I would suggest doing some reading on your own, because this is a common issue.

 

[SJones]

I think that the key point is: select a coating that has demonstrated durability and apply it properly. I have to admit to playing devil's advocate in stimulating the discussion, being aware that some coatings do lose their insulating properties over time. FBE pipeline coatings are one example. So, a good, thick film barrier coating may be the best option, possibly not epoxy based?

Steve Jones
Materials & Corrosion Engineer

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

 

[cloa]

A key factor is humidity and even more powerfully poolling water- generally needs over 80% Relative humidity for there to be a film of moisture with which to form a macro corrosion cell.

 

[unclesyd]

There is an anode and cathode involved and both are in the pit.
The electrochemistry around a pit can be very complicated and hard fathom except on larger scale.
The center of the pit is anode and edges are the cathode. One scenario is that the center of the pit is deprived of O2 while the edges are not, making the center anodic while the edges are cathodic. This condition is know as a Differential Aeration Cell or Oxygen Cell. There is another path to pitting called the concentration cell, of which I don't think is in this process, though it may have been initially.

 

[MechEng1977]

What baffles me is if pin-holes are really that detrimental, then the coating industry as we know it would not exist!

I don't buy it. In my experience at a pin hole locations the material corrodes to a surface rust and then stops corroding. There is no etching of the metal.

 

[Metalguy]

It all depends on how corrosive the environment is. Ordinary red rust (Fe2O3) is not very protective. Pinholes CAN be very bad *if* there is a large cathodic area nearby, e.g. a coated carbon steel pipe connected to a bare stainless one.

Many years ago someone at TVA, IIRC, did some experiments on "100% solids" epoxy coatings which are widely used at nuke power plants--specifically a BWR Mk1 torus internal coating.

They found many pinholes with room temp. curing in the two brands of epoxy they tested, but the number of defects decreased as the curing temp. was raised. I think they had to go up to 300F before one of the coatings cured defect-free.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[unclesyd]

Coatings live or die form the results of testing on KTA panels. One would be surprised at the number of coatings that fail. The failure rate is is much higher when a defect is introduced in the coating after application or during the test. If one takes these results and add in poor application practices you get a much higher number.
After saying all that a holiday in a coating is not necessarily bad. If you have a good primer it will suppress pitting process. Normally a pinhole takes a long to initiate but progresses rapidly once it starts. In critical applications once the coating is cured we will use a bird dog or spark to check the coating.
Based on the size of a pinhole the driving force can be quite high. I don't have access to the numbers at present. When you mention Anode and Cathode relative size doesn't is not the whole picture. Work done during the development of an electrolytic process we did a lot of capillary testing of the Anode Surface where we found small Cathodic areas on a CS Anode with a current density of 100 amps/square inch. With some electrolyte compositions the potential was around one millivolt. The big problem was finding ways and means to mitigate the process to improved the Anode Corrosion Rate.

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