Pin Hole Corrosion Acceleration
Pin Hole Corrosion Acceleration
(OP)
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
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





RE: Pin Hole Corrosion Acceleration
RE: Pin Hole Corrosion Acceleration
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
RE: Pin Hole Corrosion Acceleration
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..
RE: Pin Hole Corrosion Acceleration
RE: Pin Hole Corrosion Acceleration
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.
RE: Pin Hole Corrosion Acceleration
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?
RE: Pin Hole Corrosion Acceleration
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
RE: Pin Hole Corrosion Acceleration
RE: Pin Hole Corrosion Acceleration
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
RE: Pin Hole Corrosion Acceleration
Yes, obviously there is no metalic path through the in-tact coating, the pinholes are still anodic to the rest of the coated surface
RE: Pin Hole Corrosion Acceleration
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.
RE: Pin Hole Corrosion Acceleration
"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
RE: Pin Hole Corrosion Acceleration
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.
RE: Pin Hole Corrosion Acceleration
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04
RE: Pin Hole Corrosion Acceleration
RE: Pin Hole Corrosion Acceleration
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.
RE: Pin Hole Corrosion Acceleration
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.
RE: Pin Hole Corrosion Acceleration
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
RE: Pin Hole Corrosion Acceleration
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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