Within corrosion research, the analysis of electrochemical noise offers a simple, sensitive and virtually non-destructive measuring technique for assessment of the corrosion susceptibility of metallic materials and for the investigation of corrosion processes. The present status of knowledge concerning noise diagnostics in corrosion processes permits the application of this method not only to experimental tasks in the laboratory, but also to special problems in the context of practical corrosion monitoring. Furthermore, specific advantages of the technique enable its use to an increasing extent in supporting or improving conventional corrosion testing. The advantages here include obtaining additional information and shortening testing times (together with associated savings in resources such as manpower, energy, chemical reagents, etc.), thus resulting in state-of-the art corrosion testing.
In this paper, it is shown, with the aid of three examples taken from the routine work of a corrosion laboratory, how conventional testing methods and investigation procedures can be enriched by the application of electrochemical noise techniques. In the first example, current noise measurement using a zero-resistance ammeter is used to investigate the development of corrosion over time during salt-spray testing, thus making assessment of the test samples better and more quantifiable. During potentiostatic experiments, in the second example, additional noise diagnostics permits conclusions as to critical regions of potential and chloride concentration, as well as the effect of inhibitors, to be drawn within the shortest possible testing period. Detection of the initiation processes of corrosion (e.g. incipient pitting) is not possible simply using the conventional method of total current measurement. In the third example, the testing requirements for the determination of critical pitting temperatures can be reduced considerably and, in particular, the evaluation of the noise signals can be carried out so as to eliminate to a large extent both external and subjective influences. Furthermore, other important information, such as the incipient pitting and repassivation behaviour of the materials, can also be obtained.
There are a lot of approved and standardized methods available for corrosion testing which have found wide application. Salt-spray testing, electrochemical polarization techniques and exposure tests in specific test media belong to these. Nevertheless, some questions remain unanswered and requirements unsatisfied. In particular, both the start of corrosion and the way it proceeds with time can often be examined only to an insufficient extent. Furthermore, the requirements of practical test conditions, acceptable effort and expenditure and shorter test durations lead to continuous development of these test methods. In the early Seventies, the significance of using small electrochemical signals to examine the early processes of corrosion was recognized. This resulted in the development of novel test and investigation methods based on electrochemical noise measurements. Noise diagnostics is now regarded as a trend-setting method in corrosion research and testing. Numerous publications deal with this topic. The following examples are focused on:
Corrosion monitoring [1-8]
Testing of highly alloyed steels (pitting and crevice corrosion) [9-16]
Testing of aluminum and its alloys (pitting and crevice corrosion) [17-21]
Testing of pure iron and mild steel (pitting corrosion) [22-27]
Investigation of stress corrosion cracking [28-35]
Examination of metallic or organic coatings on metals [36-43]
Investigation of microbially induced corrosion [44-4