galvanic corrosion

[stevesxm]

new 16,000 gal concrete pool with epoxy coating using autopilot total control. water chemistry is good and all levels acceptable. have developed a few tiny rust spots coming thru the the epoxy. very very small ... a 1/16 " dia sort of thing. also the stainless pool lights have developed a bit of rust on the raw edges where they were manufactured... not on the smooth surfaces. the steel from the consctruction was all recessed and nuetralized and the entire surface plastered before the epoxy so the steel is at least 3/8 below the surface. i have used the epoxy before and it is a superb product. i spoke to the epoxy guys and they said " galvanic corrosion " ... needs sacridicial zincs etc... but..

1) other than the pool lights there is no metal in the pool at all... zero

2) the pumps are bonded and grounded.

3) all the pipework is plastic

so.. where is the potential that is causing this corrosion ? or... is it really something else.

the pool light issue doesn't really surprise me... they are chinese and their stainless sucks and often rusts by itself anyway but the pin spots comong thru the epoxy worry me.

 

[JohnT]

My guess would be that it is not galvanic corrosion, but the result of an electrical current flowing in the soil. Is the rebar in the pool structure bonded? You may want to try measuring voltage between rebar and the bond wire at the pumps.

 

[stevesxm]

My guess would be that it is not galvanic corrosion, but the result of an electrical current flowing in the soil. Is the rebar in the pool structure bonded? You may want to try measuring voltage between rebar and the bond wire at the pumps.

have no access to the rebar anymore and i have had meters between everything and evereything and find 1/10 th volt dc every once in a while but thats all.

does grounding or bonding the actual concrete do anything ?

 

[chem geek]

Grounding does not help because the ground may be at a positive voltage potential if there is stray voltage from a utility line or its (improper) ground. Also, the bonding wire is supposed to be connected to all metal in the pool, but it isn't always grounded. Putting a zinc sacrificial anode connected to the bonding wire and buried in moist soil should put a small negative voltage on the wire and that should inhibit corrosion of any metal connected to that bonding wire.

 

[stevesxm]

Grounding does not help because the ground may be at a positive voltage potential if there is stray voltage from a utility line or its (improper) ground. Also, the bonding wire is supposed to be connected to all metal in the pool, but it isn't always grounded. Putting a zinc sacrificial anode connected to the bonding wire and buried in moist soil should put a small negative voltage on the wire and that should inhibit corrosion of any metal connected to that bonding wire.

ok... i'l bite... how do you test for " stray voltage " or is that one of those unicorn/ufo myth things that takes the blame when ever everything else fails...

 

[gtemkin]

How do you know the rust spots are "coming through the epoxy"? It's quite common to have bits of iron/steel from someone having drilled a hole in an electrical box or the like make their way into the pool and leave tiny rust spots, especially on a new pool where lots of construction is taking place. The first time you vacuum the pool the evidence is gone except for the rust spot. For rust to be showing through 3/8"+ of concrete, plaster and epoxy on a new pool would mean a rate of corrosion that would be extraordinary although I'm not sure exactly what time frame you mean by "new". As for the rust on the edge of the light fixtures - which is where they were sheared in a steel punch press die - the parts may have not been properly passivated after manufacture. The sheet stainless steel they're made from is pre-passivated on its major surfaces but any edges cut with iron based tooling would need to be re-passivated to ensure no future corrosion. That may not have been done. I'd talk to the manufacturer about getting replacements.

 

[chem geek]

ok... i'l bite... how do you test for " stray voltage " or is that one of those unicorn/ufo myth things that takes the blame when ever everything else fails...

Stray Voltage from PG&E
Stray and Contact Voltage at Swimming Pools from IEEE
Stray Voltages in Swimming Pool Areas from IEEE
Stray Voltage from Bass Associates
Stray Voltage in New York
Stray Voltage from MSU
Article 680 Code
Causes, Concerns and Remediation of Stray Voltages on Distribution Systems

The increased salt level in an SWG pool increases the conductivity which increases the corrosion rate generally. Also, the higher chloride level inhibits the reformation of the passivity layer of stainless steel. If you add on top of this a negative voltage on a metal part (relative to some other area of the pool that is more positive), then corrosion of that metal will be more rapid. Likewise, if you put a [EDIT]negative[END-EDIT] voltage on the metal, it will inhibit corrosion and this is the purpose of the zinc sacrificial anode.

 

[stevesxm]

How do you know the rust spots are "coming through the epoxy"? It's quite common to have bits of iron/steel from someone having drilled a hole in an electrical box or the like make their way into the pool and leave tiny rust spots, especially on a new pool where lots of construction is taking place. The first time you vacuum the pool the evidence is gone except for the rust spot. For rust to be showing through 3/8"+ of concrete, plaster and epoxy on a new pool would mean a rate of corrosion that would be extraordinary although I'm not sure exactly what time frame you mean by "new". As for the rust on the edge of the light fixtures - which is where they were sheared in a steel punch press die - the parts may have not been properly passivated after manufacture. The sheet stainless steel they're made from is pre-passivated on its major surfaces but any edges cut with iron based tooling would need to be re-passivated to ensure no future corrosion. That may not have been done. I'd talk to the manufacturer about getting replacements.

well... new is new. i am an engineer and built the pool with my crew. the concrete set for 90 days before it was finish pastered and then it got 3 coats of epoxy after nuetralizing the concrete. no holes, no metal, no drillings. the first spots showed up in 2 weeks, the others in about a month. that was 2 mos ago give or take and nothing since. these spots are on a line where the ties would have been and they happened quite quickly but only those spots ... no others... the waterline tile was done with epoxy adhesives and the grout was epoxy grout. there SHOULD be no way that any moisture gets to the steel to rust it and no way it should come thru... but there it is. and thats the question...

 

[stevesxm]

ok... i'l bite... how do you test for " stray voltage " or is that one of those unicorn/ufo myth things that takes the blame when ever everything else fails...

Stray Voltage from PG&E
Stray and Contact Voltage at Swimming Pools from IEEE
Stray Voltages in Swimming Pool Areas from IEEE
Stray Voltage from Bass Associates
Stray Voltage in New York
Stray Voltage from MSU
Article 680 Code
Causes, Concerns and Remediation of Stray Voltages on Distribution Systems

The increased salt level in an SWG pool increases the conductivity which increases the corrosion rate generally. Also, the higher chloride level inhibits the reformation of the passivity layer of stainless steel. If you add on top of this a negative voltage on a metal part (relative to some other area of the pool that is more positive), then corrosion of that metal will be more rapid. Likewise, if you put a positive voltage on the metal, it will inhibit corrosion and this is the purpose of the zinc sacrificial anode.

i understand the theory and the concept... but in the absence of any measureable voltage at any level betwen any two of anything, where is the current flow coming from ? and what are you suggesting as far as hooking a positve lead to the pool ? ... that sounds insane to me.

 

[chem geek]

You have measured the voltage between the corroding metal and the pool water a distance away? Technically, one could also use a reference electrode to determine voltages more easily at different points in the pool and with different metals in contact with the pool.

I wrote it correctly in my first post, but in my second one I wrote positive when I meant negative (I've corrected that in the post now).

When you connect a zinc sacrificial anode electrically to other metal that is more noble including copper, iron (and stainless steel) and even aluminum, it most certainly imparts a small negative voltage. That is how it works. The zinc corrodes more readily so goes from a neutral metal atom to a positive ion where the electron is left on the wire imparting a small negative voltage. The small negative voltage inhibits the corrosion of the metal you want to protect because it increases the activation energy (or overvoltage) that must be overcome for that metal to become an ion and put its electron onto the wire. This negative voltage on the wire imparts virtually no current and it's a small voltage (less than 1 volt) so there is no safety issue.

The use of zinc sacrificial anodes in salt pools is fairly common when one has an automatic "vanishing" safety cover that uses an aluminum header bar that is in contact with the water (my electric safety cover is not "vanishing" so its aluminum header bar is not in contact with the water).

 

[gtemkin]

Could you put some photos up, maybe a closeup and then a far away shot so we could see what it looks like where in the pool your seeing this?

 

[stevesxm]

Could you put some photos up, maybe a closeup and then a far away shot so we could see what it looks like where in the pool your seeing this?

i can try but there is really nothing to see. it/they are simple tiny brown dots against a blue backround on a flat wall in about 8 places on about a 100 ft perimeter or so. i can take a piece of 600 and scuff it away and it leaves a tiny brown dot about .020 in diameter that is the source where the corrosion has come thru. and thats it. at some point i am going to try an underwater epoxy patch on it to see if it seals it but i am really looking for a root cause.

 

[JamesW]

the steel is at least 3/8 below the surface.

That's not very deep. I would suspect the issue is with the wire ties that connect the rebar at the rebar crossings.

How thick is the concrete?

 

[gtemkin]

I would suspect the issue is with the wire ties that connect the rebar at the rebar crossings.

That's seems like the most likely root cause given the small diameter of the spots and the rapidity of their generation.

 

[stevesxm]

the steel is at least 3/8 below the surface.

That's not very deep. I would suspect the issue is with the wire ties that connect the rebar at the rebar crossings.

How thick is the concrete?

there never was a question about that. but those bits of wire are , if correctly placed , are 2.5 inches below the surface of the concrete, then 1/4 " of plaster, then 3 coasts of epoxy. and if i screwed up and didn't see some poking thru
( and i hand inpsected the surface before the plaster ) then they are at least 1/4 " below and the 3 coats.. when i say " root cause " i mean how is moisture getting to it at all and b) if the only metals present are the stainless surface mount lights and i can't measure to 3 decimal places any voltages anywhere , what is causing these tie wires that are so deeply buried and waterproofed to rust ? the epoxy coating should have created an inert vessel completely insulated from the concrete.

by the way... about these sacrificial zincs.... what happens to all that zinc that is dissolved into the water ? ... and what does that do to the various sensor elements and the cell ? aren't you just creating an electroplating process and , for lack of a better term , just galvanizing everything metal that the water touches with zinc like some chain link fence post ?

 

[duraleigh]

I'm diverting a bit here but am curious why you chose to epoxy over plaster?

I agree completely on the rust on the light ring. I have seen that before

I have no educated opinion on the other rust spots.

 

[stevesxm]

I'm diverting a bit here but am curious why you chose to epoxy over plaster?

I agree completely on the rust on the light ring. I have seen that before

I have no educated opinion on the other rust spots.

its a superior product and i have used it often. impervious to everything. completely and absolutely waterproofs the surface and fill cracks if they exist, expands and contacts with any reasonable expansion or flex, lasts , essentially forever and is avaialable in any color. it has a draw back or two... it is pretty slippery and you can bleach the color out a little bit if the owner just dumps chemicals in with reckless abandon. i am not in the pool business per se but i have seen and repaired a a number of them here and its always cracks and bad grout and water incursion into the concrete because the surfaces were done badly... and that is easy to do in this environment ( carribean ) . these guys try to plaster and apply their surfaces in the heat of the day which is like working on the surface of the sun and the stuff dries too fast and cracks or doesn't bond before they are 3 feet away. plus the quality of the concrete work is genrally poor... so... on pools that i have to fix, i strip off all the old tile, grind the surface smooth and clean, a fresh coat of plaster ( at night ) and 30 to 60 days later epoxy them and they are perfect when complete. and they never crack and never leak again. its good stuff.

 

[chem geek]

by the way... about these sacrificial zincs.... what happens to all that zinc that is dissolved into the water ? ... and what does that do to the various sensor elements and the cell ? aren't you just creating an electroplating process and , for lack of a better term , just galvanizing everything metal that the water touches with zinc like some chain link fence post ?

Though there are some "inline" zinc sacrificial anode products that would indeed put zinc ions into the water, I wouldn't use them in order to avoid metal staining, though it takes somewhat extreme conditions to get zinc staining and there isn't very much put into the water (zinc staining is from zinc oxides or hydroxides at higher pH). As far as plating out zinc, that won't usually happen because there is no metal in the water with a negative enough voltage to do that -- the negative voltage the zinc puts onto the bonding wire and associated components isn't enough to overcome the overvoltage (activation energy) needed to plate zinc ions out.

As I had mentioned, you should take the zinc sacrificial anode and bury it in moist soil. It is effectively grounded so that the zinc can corrode into the soil. That avoids any potential for zinc staining in the pool.