Uniform/General Corrosion

DESCRIPTION

General attack is typically caused by uniform general corrosion. Uniform corrosion can be described as follows: Corrosion reaction that takes place uniformly over the surface of the material, thereby causing a general thinning of the component and an eventual failure of the material.

Prevention or Remedial Action

• selection of a more corrosion resistant alloy (i.e. higher alloy content or more inert alloy)
• utilize coatings to act as a barrier between metal and environment.
• modify the environment or add chemical inhibitors to reduce corrosion rate.
• apply cathodic protection.
• replace with corrosion resistant non-metallic material.

Standard Test Methods

• ASTM G-31 - laboratory immersion corrosion testing of metals.
• ASTM G-4 - corrosion coupon tests in plant equipment.
• ASTM G-54 - practice for simple static oxidation testing.
• ASTM G-59 - practice for conducting potentiodynamic polarization resistance measurements.
• NACE TM0169 - laboratory corrosion testing of metals for the process industries.
• NACE TM0274 - dynamic corrosion testing of metals in high temperature water.
• ASTM B-117 - salt fog testing.
• ASTM G-85 - modified salt spray (fog) testing.
• ASTM D-2776 - test for corrosivity of water in the absence of heat transfer, by electrical methods.
• ASTM D-2688 - test for corrosivity of water in the absence of heat transfer, by weight loss methods.
• ASTM G-91 - test method of monitoring atmospheric SO₂ using the sulfation plate technique.

Evaluation of General Corrosion

The predominant standard utilized for general corrosion assessment is ASTM G31. This standard gives guidelines for conducted simple immersion corrosion tests. Important considerations when conducting such tests in either the laboratory, field or plant setting are:

• Adequate solution volume for the surface area of corroding specimens in test.
• Electric isolation of the specimens from other specimens and any dissimilar metals in the system.
• Exposure of specimens to more than one phase, if applicable, since corrosion rates can change substantially in the different phases especially as water and impurity contents vary.
• Other test conditions such as flow rate, temperature, and aeration can produce variable results and locally high corrosion rates.

Methods of specimen surface preparation and post-test cleaning should be controlled as defined in the test standards.