What fluoride Reading should I be getting in a zinc phospate bath?

[Removed by Fluther via internal edit] Now unless you have the answer please do not comment. Unlike youself who is probably retired, I do not have as much time on my hands as I have a job to do.
I work for a very large company and have just introduced a new project in which there is not only steel but also aluminum parts in which need phosphate and electrocoating, and we are currently experiencing some problems with the aluminium not getting a sufficient coating of zinc.

Don’t know why you selected Zn unless yo needed a sacrificial coating. I’m sure there is a good reason. Temp, flex, chemical.
Have you looked at cathodic epoxy electrocoating? You can do the steel and Al at the same time in situ.

We have used Zn for about 20years. Up until now we have just been coating steel. The temp of the bath is at a controlled 56 degrees celcius, the electro coat has always been fine therefore this is why we are investigating into why the zinc isn’t giving us a good layer on the alumium, and we have come down to the free fluoride content in the bath. Cheers

There is a paper by J.B. Bajat et al that might apply. Here is an excerpt from the abstract. :

“The influence of hot-dip galvanized steel (HDG) surface pretreatment with phosphate coatings on the corrosion stability and adhesion characteristics of epoxy coatings electrodeposited on HDG steel was investigated. Phosphate coatings were deposited on hot-dip galvanized steel from baths with different concentrations of NaF (0.1, 0.5 and 1.0 g dm−3) and at different temperatures (50, 65 and 80 °C). The influence of fluoride ion concentration in the phosphating bath, as well as the deposition temperature of the bath, on the adhesion characteristics and corrosion stability of epoxy coatings on phosphated HDG steel was investigated…... The highest values of adhesion strength were obtained for epoxy coating on HDG steel modified by phosphate coating deposited at 65 °C with 0.5 g dm−3 NaF in the phosphating bath. EIS measurements in 3% NaCl solution are in accordance with adhesion measurements, i.e. greater values of charge-transfer resistance and smaller values of double-layer capacitance were obtained, as compared to other protective systems.”