Standard Test Method for Monitoring Atmospheric Corrosion Tests by Electrical Resistance Probes

Importancia y uso:

4.1 Corrosivity monitoring of test environments provides a means to monitor an integrated value of test corrosivity which cannot be evaluated from test parameters themselves, such as temperature, humidity, and gas concentration. As such the monitor value can be used for specification purposes such as test validation. Electrical resistance monitoring of conductors exposed to corrosive media is a well-established practice.3^,4^,5^,6

4.2 The resistance method assumes uniform corrosion over the entire surface of the exposed metal conductor segment. Local corrosion such as pitting, crevice, or grain boundary corrosion may provide invalid estimates of test corrosivity. Marked changes in slope of the curve of electrical resistance ratio versus time may indicate undesired processes which can be due to deficiencies in the test atmosphere or in the monitor itself.

4.3 Because of limitations of the diffusion process within the corrosion product formed on the metal conductor segment of the RM probe when passivating corrosion films are formed, resistance monitoring may not be useful for test chamber monitoring purposes for very long test exposures. Chamber monitoring is dependent on detecting changes in the rate of corrosion of the RM as an indicator signal that specified gas concentrations must be reverified. However, low corrosion rates limit the absolute value of the rate of change of corrosion rate with change of test conditions; for parabolic film growth processes, the growth rate decreases with time limiting the sensitivity of the RM at extended test times.

4.4 Since corrosion rate can be a complex function of test parameters in MFG tests with any given metal primarily responsive to a subset of the gases in the MFG environment, more than one type metal resistance probe is required in order to assist in maintenance of relative gas concentrations. For such test specifications, values of resistance ratios must be referred to ratios obtained under known test conditions as supplied by the test specifier. Information relating to the sensitivity of various metals to various corrodants has been published.7^,8

4.5 RM probes can be useful from 1 % of thickness consumed upward to 50 % of thickness consumed by the corrosion film growth. Conductor thicknesses between 25 nm and 0.2 mm have been reported and common sizes are available commercially.

Subcomité:

B02.05

Referida por:

B0827-05R20, B0845-97R24

Volúmen:

02.04

Número ICS:

77.060 (Corrosion of metals)

Palabras clave:

atmospheric corrosion monitor; atmospheric corrosion testing; corrosivity monitor; electrical resistance probe; environmental test; mixed flowing gas test; resistance monitor probe;