Mass Loss (Weight Loss) Test

Overview

One of the oldest methods of testing the corrosion resistance of materials is to expose that material directly to the actual or simulated environment. The alloy specimen, often referred to as a coupon, is weighed before and after the test. Though the technique is conceptually simple, significant care is required in the handling of the specimens and the experimental procedure used for the exposure. The prediction is only as good as the ability of the test to duplicate the actual exposure in the environment.

Experimental Technique

The technique is amenable to both laboratory and field (plant) exposures. One type of specimen commonly used for such immersion is a rectangular shaped specimen, often between 0.125 inch and 0.25 inch (3.18 mm to 6.35 mm) thick. Very often, a weld is made along the center of one side so as to be able to assess selected attack of the weld. The common calculation of corrosion rate does not single out such selective attack.

An example of the rectangular specimens sometimes used is shown below.

These particular specimens were exposed in a vessel so that one specimen was fully immersed in the liquid phase, one specimen was placed at the vapor-liquid boundary, and one specimen was placed in the vapor phase. Shown below are examples of specimens and specimen holders that have been used for laboratory and field exposures.

When metals or alloys are being evaluated, the change in mass is obtained by weighing the specimen before and after exposure and converting that mass change during the exposure period to a penetration rate. Further information on the calculation as well as an on-line calculator can be found at http://www.argentumsolutions.com/tutorials/calculator_tutorialpg2.html. Significant information on such topics as specimen preparation, specimen cleaning, and appropriate experimental techniques can be found in ASTM G1 "Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens", ASTM G4 "Standard Guide for Conducting Corrosion Tests in Field Applications", and ASTM G-31 "Standard Practice for Laboratory Immersion Corrosion Testing of Metals", all in Volume 03.02 Corrosion of Metals; Wear and Erosion, ASTM Annual Book of Standards and available at http://www.astm.org.

Implicit Assumptions

  • The penetration is uniform or even across the specimen surface. No areas of greater and lesser attack are observed.
  • No localized corrosion such as Crevice Corrosion or Pitting Corrosion occurs.
  • The projected and average surface areas are the same. The actual surface area is determined by the local contours of the surface. A smooth surface will have much smaller difference between the "peaks" and "valleys" than a rough surface.
  • Coupon area (coupon dimensions) is unchanged during exposure. The usual procedure is to measure the corrosion coupons before exposure and assume those dimensions when estimating the surface area.
  • Weights are unaffected by corrosion product removal procedures.
  • The penetration rate does not vary during the exposure. The time used in the calculation is usually the total exposure time for the given specimen. Corrosion rates are sometimes not constant. They can change as corrosion proceeds, decreasing in some cases, increasing in others.
  • Calculation errors are not propagated because of the uncertainty in the measurement of time, mass, and dimensions.

Further information on these assumptions can be found at http://www.argentumsolutions.com/tutorials/calculator_tutorialpg2.html.

Return to index page of CORWIKI.