Citralic acid on cedar shake w/ galvanized nails

[epictetus 14]

The cedar shake on a friend's house (currently under construction) was treated with a sealant and it became a little too dark, so he had the contractor who applied the sealant lighten it up. They sprayed it with citralic acid, which I've just learned is a mixture of oxalic and citric acids and it did lighten the wood up but it caused a black stain to appear around some the galvanized nails in the shake. The nails were electroplated galvanized as opposed to "hot dipped." Any suggestions as to why this happened and how he should go about fixing it? Thanks.

[Beth n Rod 1,279]

One of three things could be going on here:

#1-The acid may still be active contributing to #2

#2-The nails could be rusting from age and exposure to acid rain, the treatment or a combination of both. (depends on the strength and dwell time)

#3-The tannin (an extractive) is still bleeding from the wood. (most likely)

The one suggestion I can make is to re-treat the areas again and once the acid has been rinsed after bleaching the stains, the same areas should be sprayed with a solution of Sodium Bicarbonate to neutralize the acid. This will keep the acid from being reactivated from humidity or rain.

There is still a possibility that the stains could reappear in time anyway as this is normal with softwoods such as cedar and redwood.

The galvanization process or either electroplating or hot dipping is of no consequence in this case as the cedar shake has no additional preservatives which could lead to fastener corrosion. Pressure treated woods having ACQ, CA etc. can have an detrimental effect on electroplated fasteners.

Rod!~