Expert: Craig - 9/6/2007

Question
QUESTION: Hi - first of all thank you for helping me out.
We are involved in a building where the pool (21K gallons of water) diamondbrite finish has rust like stains in random locations. This problem has been brought to the attention of the pool contractor, developer, general contractor and current pool maintenance contractor. The pool has been previously acid washed but the stains come back up. At this time we have not been able to eradicate this problem and although we have a good idea of what is going on, I would appreciate if you could stir us in the right direction.
We have performed water tests to find if there were traces of metal in the pool along with the usual tests of Chlorine, pH, Alkalinity, Calcium Hardness, Stabilizers and TDS. The test results showed no traces of metals in the water, low Alkalinity, low (zero) stabilizer and high TDS. In addition to this, there are low levels of sequesterants (2 ppm).
We also performed lab tests on a piece of the diamondbrite finish and there are traces of Copper and Iron and it was suggested the iron is likely from the water and the copper from either (1) use of water treatment containing high copper (ei. Algaecide) or (2) corrosion of a copper pipe.
We have contacted the pool heater manufacturer (Jandy) to see if they can narrow down what chemical may be reacting with the copper piping or provide us different solutions.
One again, thank you for your help.

Anthony

ANSWER: Anthony:

First,do you find the stains reappearing in the same spot?  Also, do you find that the stain(s) are concentrated (darker) stain?  If so, my bet is that the pool builder did not apply a thick enough coating of gunite over the rebar.  The rebar is rusting causing stains to bleed through the diamond brite.  The only way to fix this is to chisel down to the rebar, sand away the rust, apply a rust inhibitor on the rebar,  reapply a concrete patch and then replaster (veryexpensive repair job).

If the stains are not concentrated, my guess is that metals are in the water. I would look at your plumbing first.  Is your system plumbed with PVC pipe or copper/galvanized pipe?  I usually replace all metal piping with PVC.  

Finally, the heater has copper tubing inside of it.  If the pool's chemistry is off, the copper will break down causing metals to enter the water and stain the surface. Eventually the heater exchanger will develop a leak which is very costly to repair and usually requires the replacement of your heater.  Make sure your readings are as follows:

Chlorine 1.5-3ppm, pH 7.6, Alkalinity 100-Lapp, Water Hardness 230-270pp.  

I would also test the fresh tap water for metals and compare it against the water in the pool.  If they are the different, then you know the staining is a result of your pool (rebar or piping).

You can add Stain/Scale/Metal remover.  Usually comes in QT size containers.  Follow the directions on the bottle.

Before I forgot.... you should avoid acid/chlorine washing pools at all costs.  This will corrode at your surface and make it rough and reduce its useful life.

Good luck,

Craig





---------- FOLLOW-UP ----------

QUESTION: Once again thank you for your help and quick response.
To my knowledge the stains appear in random locations of the pool and are not that concentrated to suggest exposed rebar throughout. We did have some locations were it was visibly exposed but it was already repaired and the staining removed. Due to other problems (ei. surface spalling) we are planning to empty the pool and tap the surface to find all voids (hollow areas) and then we will be able to check if exposed rebar may be the case.
The piping of the system is all PVC and the only copper piping is within the water heater, which could be the source of the traces of copper found in the water/surface. However, it is my understanding that copper will not corrode in a rust like color, but greenish or grayish.
When tested the chemicals were the following:
Free Chlorine 4.0 ppm
pH 7.4
Alkalinity 55 ppm
Calcium Hardness 250 ppm
Stabilizer 0 ppm
TDS 5,000 ppm
Phosphate 500
Sequestrant 2

The acid wash was recommended by the pool installer and they did performed the wash.
I will collect the fresh tap water and send it for testing. I will let you know exactly what we find.
Once again thank you for your help Craig,
Sincerely,

Anthony

Answer
Anthony:

Spalling is often caused by rusting rebar.  It is very likely that could be your problem.  Make sure it is fixed properly or the same thing will happen again.

Also, your pool is too acidic.  I know that many pool guides will say that 7.4 is okay but that is not good enough for me.  7.6 is perfect and that is where it should be.  Also, your alkalinity is way too low. It should be 100-120ppm. Add sodium bicarbonate (baking soda to bring it higher).  You will be amazed that proper alkalinity will keep the pH at 7.6.

Also, you should add stabilizer to your pool.  Chlorine in its natural state will dissipate into the atmosphere without effectively sanitizing your pool.  A cyanuric acid reading of 20-40ppm will keep the chlorine in the water long enough to effectively kill bacteria and algae.

You are correct that corroding copper will create a greenish tint.

Regarding the acid wash by your pool builder.... it is a quick fix and it will create immediate results. However, it will reduce the useful life of the surface. It's kind of like.... not waxing a car.... your car will look good but the finish will last a lot longer if you routine wax it.

I assume your skimmer and drain are PVC as well. What about your light.  Usually it is stainless steel but.... I have seen cases where the electric company has an electrical transformer nearby on your properly and it leaks electrical current underground.  This will cause the stainless steel to corrode as well.

Good luck,
Craig