Iron Contamination of an ED Bath


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What is being measured?

I have attached a copy from Hach Instruments, which deals with the measurement of iron. I do not have a copy of PPG’s iron test method and will guess they are looking at soluble iron (Fe+2 and Fe+3) levels in the ED bath.

How does soluble (and insoluble) iron get into the ED bath?

It comes from sources that 1) contain iron, 2) may be an anode (I am not talking about what is connected to the anode bus bar of the DC rectifier, but rather an anode of some small battery-like circuit that also has a cathode), and 3) has direct contact with the ED bath. I have listed these from large contributor to small contributor.

Take a closer look at iron sources.

  • Electrode (anode) of TECTRON Bare Floor Electrode. . The iron of any stainless steel (alloy) anode is sacrificial under the influence of a voltage gradient. Under the best of situations, 316L stainless steel erodes at a rate about 5 – 10 x 10-6 gr/Coulombs. The theoretical limit is about 220 x 10-6 gr/Coulomb, however actual loss rate can be even higher due to disintegration of the stainless substrate. All of the iron is released into the ED bath.
  • Electrode (anode) of side box cells Generally any iron that goes into the anolyte solution is removed from the box cell by the flow of the anolyte. As the ion-exchange membrane ages, gets cut or torn, some of the anolyte is allows to pass into the ED bath.
  • Electrode (anode) of TECTRON Floor Cell. Same as above.
  • Another contributor is iron filings and weld debris that is coming off the vehicles and all this iron material can be carried over into the ED bath.
  • Some pretreatment systems use iron phosphate. I believe you use zinc phosphate. Certainly, there can be carry over from the pretreatment system.
  • Is the bottom stainless steel grid (job off detector) grounded, or does its voltage float in the ED bath? If its voltage is floating, then a part of the grid may be acting as the ‘anode’ and another part is acting as the ‘cathode’. Under this situation, there will be erosion at the anode, which will release iron into the ED bath.
  • All of the clamps (304 SS) that secure the Floor Cells, Electrodes, side rub rails, and crash bars are not grounded and thus have a floating voltage. See above regarding grid.
  • There are other smaller static sources such as the stainless hardware on the anode box cells, and other stainless piping & valves. See above regarding grid.

Where is iron used?

Not all the iron stays in the ED bath. Since it carries a +2 or a +3 state, it will be attracted to the cathode. In this case the iron is embedded in the ED paint. I do not have an estimate of how much or how fast soluble iron is able to migrate in the ED bath, but I suspect it is rather fast on its feet and pretty mobile.

Why is iron a problem?

At a certain point, the amount of iron in the bath becomes significant. Thus iron then becomes a significant portion of the ED film. The visual defect you will then see is called ‘orange peel’ because the cured ED film has rough surface. The iron interferes with the curing and does not allow the ED film to flow together properly.

MMMA Iron PPM Trend Chart.

There has been an elevated iron ppm level starting about the same time as the installation of the Bare Floor Electrodes (April 97). It then peaked at 37 ppm and has gone downward since the first 4 Bare Floor Electrodes were disconnected and the first 8 Bare Electrodes were converted to Floor Cells (flushable). Since mid February, the slope of line is steep about inline with the slope of the line back in April 97.


The steep slope suggests a major iron contributor is at work now. It does not take much iron to create a 25 PPM condition in your 71,000 gallon ED tank. As a rough estimate, if the densities of the ED bath and soluble iron are the same: then it would only take about 6.7 kg of iron to equal 25 PPM in an ED bath of 71,000 gallons (268,763 liters) Also on the Excel sheet is an estimate of the iron loss (assuming all of the stainless alloy is iron, which is not the case) based upon the input of givens. As you can see for the givens I have started with, the remaining 16 Bare Electrodes (over a 90 period) have lost about 29 kg, which is equivalent to about 110 PPM of iron. If indeed this is true, then a lot of the iron is being removed in the ED film. Note the original mass of one of the Bare Floor Electrodes was about 11 kg. I would begin looking & attempting to confirm the Sources of the iron in the ED bath. The first places I would look are –

  • Iron filings, or weld debris carry over from pretreatment.
  • Do I have any box Cells with old ion-exchange that may be cut, torn, or just passing more anolyte now than last year at this time.
  • If the Bare Floor Electrodes are still delivering current, then these are a known Source of iron. As before, you can disconnect a couple and check ED film builds to see if there has been a reduction in the rocker panel area.







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