Corrosion Analysis & Testing

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Pitting Attack in Stainless Steel

Pitting Attack in Stainless Steel

Stress Corrosion Cracking in Stainless Steel

Stress Corrosion Cracking in Stainless Steel

Pitting Attack in Copper Pipe

Pitting Attack in Copper Pipe

Metallurgical Consulting has over 30 years of experience in evaluating corrosion and developing corrective actions in the process industries. This experience ranges from simple failure analyses to large corrosion testing programs.

DC Electrochemical techniques have been used over most of this period to rapidly acquire corrosion data in actual plant environments and bench scale laboratory tests. More recently AC electrochemical tests have been included to evaluate non-metallic coatings for their corrosion protection ability.

The ability to acquire live time corrosion data under process conditions has enabled us to identify the specific point in batch reactions in which stress corrosion cracking is occurring. Very brief but very severe cracking has been identified in start-up conditions with these tests. This knowledge has enabled operators to modify start up conditions and either save the reactors or extend their life. The approach is briefly outlined below:

• A sample of the alloy has its potential relative to a reference electrode changed continuously or scanned from a negative or cathodic voltage to a positive or anodic voltage in the solution of interest at different stages of the reaction. The current (corrosion rate) and voltage are recorded. This can be done in a highly controlled laboratory set up or in real world conditions. (Metallurgical Consulting currently use a Gamry Ref 600 system.) The results are plotted as shown in the graph below.
• Stress Corrosion Cracking (SCC) in active-passive metals such as stainless steels is known to occur under specific electrochemical conditions as shown in the graph.
• The nature of the resulting current vs voltage plot and where the metal potential lies provides the data needed in order to determine when the cracking occurs.

The size and location of the passive region is affected by many variables including solution composition, pH, and temperature. Handbook corrosion tables cannot cover all of the issues for plant applications. However, DC potentiodynamic corrosion testing can provide data on the nature of the corrosion conditions for specific situations.

Instances have been observed where stainless reactors started out in the active region and then went passive. They must cross through a region of SCC in order to do this. Adjusting start up conditions to allow the system to start up in the passive region has reduced or eliminated the cracking.

Pitting can be addressed by potentiodynamically scanning the specimen into the transpassive region and then reversing the scan. Pitting conditions will cause the reverse current to stay high. The extent of the return hysteresis will say much about the pitting conditions. This type of test was used with side loop specimens on a once through river water heat exchanger to monitor the performance of ferrous sulfate additions to inhibit pitting corrosion. See paper. (PDF - 5.12MB)

Corrosion rates can also be measured electrochemically since the current density is directly related to corrosion rate by Faraday's law. The corrosion rate on stainless steels should be very low in the passive regions. However, rates as high as 5.0 inch per year have been measured in the active region. These rates persisted for a short time during start up only but when all startups were totaled, the total wall loss estimated from corrosion data came close to the actual measured wall loss.

AC Electrochemistry or Electrochemical Impedance Spectroscopy (EIS)

AC Electrochemistry or
Electrochemical Impedance
Spectroscopy (EIS)

AC Electrochemistry or Electrochemical Impedance Spectroscopy (EIS)

Non-metallic coatings are applied to steel and other metals to protect the metal from corrosion. A good coating provides a barrier to the flow of current, ions, and water through the coating. The coating forms a capacitor circuit when the steel is exposed to water by submersion or atmospheric water. Measuring the response to AC current of a coated specimen immersed in the environment of interest will yield extensive information on the coating and its barrier ability. The impedance or resistance to AC current flow at 100 mHz is becoming recognized as an indication of coating failure. When values approach 10 KOhms, the coating has become relatively electrically conductive because of water ingress and has started to fail. These tests also can measure the performance of zinc rich primers. The picture below shows a typical EIS coating test set up. (The steel box is a Faraday cage to shield the test specimens from stray current interference.) These tests are capable of rapidly evaluating coating behavior. They give the opportunity to look through the coating layers to see how they perform.

Corrosion Analysis & Testing Capabilities

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