A fast automatic data acquisition (33KHz) and processing system has been developed to perform nondestructive measurements of corrosion rate using the Coulostatic method. We evaluated five organic coated steel samples in contact with NaCl 3.5% P/W and Sodium Citrate (pH 3.5). The results were compared with results obtained by salt spray test performed to ASTM B117 and DIN 50907. The corrosion rate was indirectly calculated from the intrinsic Metal Charge Transfer resistance of an equivalent second order series cell electrical circuit model, as determined by an algorithm based on the Two Slope method, We were able to measure resistances in the order of up to 1010 ohm. Consequently, it was determined that the Coulostatic method is applicable in the evaluation of high impedance coating systems.
1NTRODUCTION
The evaluation of corrosion in organic coated metals and of steel reinforcement for concrete structures has a tremendous economic importance, i.e. protective paint evaluations and transportation structures maintenance programs.
Most of the nondestructive electrochemical methods used for measuring corrosion rate, perturb the Sample Under Test (SUT) This makes the generation of valid equivalent cell Electrical Circuit (EC) models and the accuracy verification of the associated results difficult, Other nondestructive electrochemical methods present serious limitations, Some methods can not be used to determine all of the components in the equivalent cell EC, Others methods take too long in taking measurements and can not be used when the electrical characteristics of the SUT are changing rapidly, AC Impedance method, although popular, is an evasive method with limited accuracy when measuring high impedances, Typical measurement lines are long - in the order of hours. In addition to these facts, the technology used on most of these techniques do not permit an inexpensive and convenient way of performing test,
The Galvanostatic Pulse and the Coulostatic methods are becoming more attractive for several reasons. They are nondestructive, non-perturbing tests. They are relatively fast and applicable when the electrical characteristics of the SUT are changing rapidly. As the technology advances, smaller and less expensive acquisition and processing components will be becoming available to enhance the capability of related instrumentation, As well, some theoretical, experimental issues and smarter processing algorithms are being defined for both methods. In principle, a certain amount of electrical charge is transferred into the SUT such that the induced voltage transient response does not exceed values c f 30mV, in order to comply with the Stern-Geary criterion of non SUT perturbation. The corresponding; voltage discharge (Coulostatic method) or voltage charge (Galvanostatic Pulse method) response is p recessed for extracting the individual components c)f equivalent ECS, An inherent limitation of the Galvanostatic Pulse method is the fact that the total charge transfer to the SUT occurs at the end of the voltage transient response. Presently it is difficult to measure that charge accurately, especially for ECS with long time constant responses. If the constant current source used has speed limitations that could make it unsuitable for ECS with short time constant responses.