Aim and Results of CMT Measurements

There is an obvious need for single, fast, and rehable corrosion measurements on metallic products, whether use in particular environments is the concern, or whether the application of a particular post-treatment necessitates a control. Various types of chamber tests are used for technical testing. These tests are accelerated, which means that the metallic items are exposed to more aggressive conditions than they are expected to meet during actual savice, but it still takes a long time to obtain a measurement. The measurement indicates the time to failure (to a certain degree or 

The CMT method was primarily developed to replace traditional corrosion tests with a faster, quantitative test. Since the course of corrosion with time is continuously shown on the screen, it is easy to see if and when a stable situation has been established, resulting in a constant rate of corrosion. The rate of corrosion at that stage can often serve as a safe measure of the lifetime of the object. An assessment of quality on the basis of such a measurement can be made in a much shorter time than by a chamber test. In the latter, a good quality product should typically have a time to failure of more than 100 h, whereas a CMT test of a similar product mostly shows a stable, constant rate of corrosion (on the order of 5-10 fiAJcrn ) after less than 2 h. 

A great number of measurements have been reported for articles electroplated with zinc. The various aims have been evaluation of the corrosion rate of zinc that had been plated in a number of commercial cyanide-free zinc baths, comparison of the corrosion rate of a composite material (zinc with codeposits of various oxides) and of pure zinc deposits, corrosion testing of various alloyed zinc platings (Zn-Ni, Zn-Co, Zn-Fe), with or without subsequent post-treatment. Most of the work in the last category was only recorded in internal reports. The published work consists of an examination of the corrosion behavior of a ctoomated Zn-Fe 

The work described above provided the technical information for which the CMT method had been developed, while various details of the instrumentation and programming were improved. However, it became obvious at an early stage that there is very rarely agreement between CMT and electrochemical measurements, even if the two are recorded simultaneously. The reliability of the CMT measurements has been tested in several cases (see Refs. 2 and 13, for example), where a sufficient amount of metal had been dissolved during an experiment. Analysis of the cell solution has then in most cases agreed very closely with the result according to the CMT measurements (the total amount of acid used in the experiment is displayed on the autoburet). 

The disaepancies between CMT measurements and electrochemical measurements have given rise to some closer examinations revealing a number of different causes ranging from trivial to novel phenomena. 

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