Method of monitoring constituents in conversion coating baths

A method for monitoring the constituents present in a conversion coating solution used in a non-electrolytic conversion coating process wherein a conversion coating is applied to a metal substrate, said method comprising the steps of:

• (a) applying a selected dc potential to a working electrode which has been subjected to annealing and anodic pretreatment, said working electrode being positioned within said solution containing said constituents, said dc potential producing a dc current;
• and wherein hydrogen gas is produced at said working electrode and said conversion coating precipitates at said working electrode;
• (b) superimposing a constant ac voltage on said dc potential applied to said working electrode, said ac voltage having an amplitude and a frequency and producing an ac current;
• (c) varying said dc potential at a chosen sweep rate over a chosen range;
• and
• (d) measuring said dc and/or said ac current or a harmonic of ac current at one or more reference phase angles with respect to said constant ac voltage between said working electrode and a reference electrode positioned within said solution as said dc potential is varied over said range, said measurement of dc and/or ac current in relation to varying dc potential being expressed as a dc and/or an ac current spectra which is used to determine and monitor said constituents in said solution.