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GM9540P

A standard for the prediction of cosmetic corrosion on cold rolled steel

Introduction

In the early 1990’s a group from the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI) evaluated number of accelerated corrosion tests, they concluded that GM9540P provided the best prediction of cosmetic corrosion of cold rolled steel.

This study is a key reason why GM9540P became popular as an accelerated laboratory corrosion test for steel.

In recent years GM9540P has been replaced by its successor; GMW14872, but this is all but identical in terms of the test conditions required.

The GM9540P test is also referred to as a Cyclic Corrosion Test, often abbreviated to CCT.

Test Conditions

The test consists of alternating wet and dry cycles with precisely prepared salt solution.  NaCL, CaCL2 and NaHCO3 are utilised for the wet cycles with precisely controlled heat and humidity for the dry cycles.

Method

Test specimens in the enclosed chamber are exposed to a changing climate that generally comprises of the following 3 part repeating cycle (variations to this cycle are permitted depending on the type of component under test – refer to the full test standard for further guidance).

  • 8 hours exposure to an ambient climate of 40 to 50%RH at +25C, during which salt solution is sprayed directly on to the test samples 4 times, for long enough to thoroughly wet them, and at intervals sufficient to allow the samples to dry between each spray.
  • 8 hours exposure to a water fog humidity climate of 100%RH at +49C.
  • 8 hours of air drying in a climate of <30%RH at +60C.

The number of cycle repeats and therefore the test duration is variable.

Typical Application

Originally developed by General Motors the standard has gained popularity for corrosion tests within the automotive industry and has also become the preferred corrosion test of the US Navy.

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