Electrolysis Question

I don't think sacrificial anodes are going to help you.

I don't know if the roofing is galvanized or painted, but the fact is that every place there is a screw hole, the roofing has been compromised. Over the years the rubber washers dry out, shrink, or just get loose from expansion and contraction of the building.

What you need to do, is climb up there, clean up the rust, tighten the screws if they are loose, hit the corrosion with some rust oleum or something similar, and then follow up by putting a dab of exterior silicone caulk or metal roof patching compound around the screw heads.

That'll do it.

the sheets are painted on both sides. the rubber washers are still in good condition. after removing a few screws, i discovered that the sheet is turning into a fine white powder starting from the screw shank and working outward. it's not iron oxide that i'ld expect to see. it looks more like battery corrosion.

Have you contacted the manufacturer?

Stop and look at the composition of all the materials you're dealing with. Disregard the rubber washers and/or any insulative materials you may encounter.

Dissimilar metal corrosion sounds as though it is the culprit. At some point one of the metals is sacrificing itself to the other. You need to identify the metal and plan on a very involved fix for the corrosion. Sacrificial anodes will not cure this problem!

Dear Guest,

Your question leaves a lot to the imagination without more detail!

You need to recognize some basic principles of corrosion in order to arrive at a solution.

The visual evidence of corrosion you mention, comparing it to what is seen on a "battery terminal", is" Hydrous Ferrous Oxide"! A less noble metal is returning to it's original state. This is not as important as it to understand this condition only occurs with the following events being in place.

1 There are two or more dissimilar metals with different voltage potentials.

2 There is an electrically conductive path that is allowing the transmission of these minute voltages.

3 Ions from the more Anodic [less noble metal] are moving to the more noble metal [Cathodic] site.

A check with the manufacturer may get you pointed in the right direction, but also understand a different batch of a particular metal may have been used from week to week at their facility. The slightest deviation in batches will make a big difference!

You do not mention the age of the concrete slab on which this facility is constructed? It could very well be that with age and high moisture concentrations in the concrete, the reinforcing steel within the foundation is responsible for Stray Currents attributing to your problem. Keep in mind that when dealing with different metals and construction techniques, various attempts are made for mechanical attachments. Attach points that are plated, painted, or insulated may work well in the short term but eventually fail.

In any event, care should be exercised in whatever fix you decide to employ. As stated previously -- sacrificial is certainly not the way to proceed! A worse case would be to employ an impressed cathodic method and watch your building crumble before your eyes.

great info. would grounding the building at different location cure the problem? i should have mentioned that this only accures on the roof. this leads me to believe water is the source of the problem. these roofs have very little pitch and water accumulates around the screws in question.