SEM

RESUME

C. Kendall Clarke
Ph.D., P.E.

President and Principal Engineer

Don Halimunanda
M.S., P.E.

Mechanical Engineer

CORROSION

Several batch reactors suddenly developed extensive cracking.

Metallurgical Consulting has extensive experience with corrosion in the paper and chemical industries.

Identifying and correcting corrosion problems in these areas requires a plant visit to see how the equipment is operated and under what conditions. A thorough understanding of the process flow, relative to the corroded equipment, should be determined.

Maintenance records are reviewed for corrosion problems. An inspection of equipment upstream and downstream of the effected equipment is frequently conducted. This information helps to scope the problem and sometimes uncovers more extensive damage than was thought to exist. The background information also usually provides insight into setting up corrosion studies to identify the problem.

Analysis of a corrosion problem can involve several steps:

Tubercules in heavy corrosion scale were identified as the result of biological attack.

• Samples of the corroded areas are usually obtained for metallographic evaluation.

• Process solution samples for corrosion tests are also obtained.

• Some corrosion problems can be identified from the metallographic evaluation and laboratory corrosion tests.

• Other problems are more subtle and require more extensive testing.

• A comprehensive test program is then developed based on the results of a preliminary evaluation.

Corrosion tests are run using a potentiodynamic technique

Corrosion tests involve a corrosion probe that can be inserted directly in the process stream or used under laboratory conditions. The ability to test in the plant is helpful where process conditions change or where the chemistry is unstable or hazardous.

Potentiodynamic tests involve subjecting a sample of the alloy to a constantly changing corrosion condition in the solution of interest by varying the specimen potential electrically. This results in corrosion rate estimates and a measure of passive film stability. Results have been used to identify stress corrosion cracking and pitting corrosion problems.

Some examples of projects in corrosion follow:

Stress corrosion cracking at a weld in 304 stainless

• A group of batch reactors developed cracking after a few years of operation. On line potentiodynamic testing revealed initial start up conditions over a short period of time were causing the cracking. Modifications to the start up could reduce or stop the cracking.

• A solvent storage tank developed extensive corrosion scale and corrosion. Extensive work on an earlier identical problem had produced evidence for iron bacteria setting up micro habitats – the tubercules.

• Extensive cracking in a 304 stainless piping system was investigated. Review of the wash water operating system revealed excessive temperature and low pH for the amount of chloride ions in the system.