Corrosion testing provides us valuable information that can be very helpful in preventing costly damage or premature failure of components and structures. Survey done by NACE International shows that about $300 billion are lost by the US Economy due to corrosion.
Corrosion can be detrimental if not diagnosed or detected timely. Failures due to corrosion can be harmful to humans, personnel property and also the environment. To avoid losses of this nature, corrosion testing should be performed in every kind of industry from refineries to utility companies.
The adage goes that best treatment is preventive. It very much applies to corrosion prevention programs.
Corrosion testing can be broadly classified into four parts:
- Laboratory tests
- Pilot plant or semi-works tests
- Plant or actual service tests
- Field tests
Why do we need corrosion testing?
Corrosion tests are performed for four main reasons which give us an accurate and precise result for the following reasons:
- Control of corrosion resistance of the material or corrosiveness of the environment.
- Study of the mechanics of corrosion or other research and development objectives.
- Evaluation and selection of a metal or alloy for a specific environment or a given definite application.
- Evaluation of new or old metals to determine the environments in which they are suitable or not suitable.
CORROSION TESTING AT AEIS
AEIS has a fully equipped Corrosion testing lab where a variety of tests are conducted for determining the corrosion resistant of materials, evaluation of corrosion prevention coatings/platings , determining the extent of Corrosion damage and also Field Survey of Corrosion Prevention Systems like Coatings , Cathodic Prevention.
UNIFORM CORROSION rate determination is one of the basic tests carried out as per ASTM G-1 where the Corrosion rates are determined using “Weight-Loss Method”. The scope of this covers Iron/Steel, Al/Al-alloys, Cu/Cu-alloys, Mg/Mg-alloys, Ni/Ni-alloys, Pb/Pb-alloys, Zn/Zn-alloys, Tin/Tin-alloys and Stainless Steels. These tests can be conducted both on Standard Coupons and actually corroded Components.
INTER-GRANULAR CORROSION (IGC) is basically localized attack along the Grain Boundaries of a metal , the typical example being the “Sensitization or Weld Decay” of Stainless Steel where Chromium-rich Grain boundary precipitates leads to areas around them becoming depleted in Chromium forming a sort of “Concentration Cell” which leads to selective attach of these areas.
We conduct various typesASTM A-262 Practice A (Oxalic Acid Etch.) , Practice B (Streicher Test) , Practice C (Huey Test), Practice E (Strauss Test) and Practice F(Copper-Copper Sulfate-Sulfuric Acid). We also use ASTM G-28 for Nickel- and Chromium-based alloys – this test is conducted for components in the power plant sector. The other IGC test conducted at AEIS for Stainless Steel are ASTM A-763 & A-923.
For Detecting IGC in Aluminum-alloys we conduct tests as per ASTM G-67, which are generally carried out on aerospace components.
STRESS CORROSION CRACKING (SCC) occurs when the materials cracks due combined effect of Corrosive environment and a Tensile Stress acting on it. Typical failures due to SSC are seen in Pressure Vessels, Pipes and other Stressed Components. The stress may be due Service stress or residual stress produced during manufacturing. Due to criticality of components and catastrophic nature of this failure it is very important to screen materials for SCC before putting them into service
We have conducted tests to screen alloys and assist in material selection for environment and service condition which are susceptible to SCC. Some of the common tests carried to detect susceptibility to SCC are ASTM G-30 (U-bend specimens) G- ASTM G-36 (boiling MgCl2), ASTM G38 (C-ring specimen test),
PITTING CORROSION is a much localized form of corrosion where the environment affects the material forming Pits or Cavities, which affects the Structural Integrity of the component.
Our engineers use a variety of test methods to determine whether a material will be subject to Pitting corrosion (Material Selection) and also perform tests to determine the extent of Pits (Residual Life Prediction). We conduct both electrochemical; tests and Immersion tests to study Pitting Corrosion. Some of the common tests we perform are ASTM G-46 (Examination & Evaluation of Pitting), G-48 (Pitting resistance) and ASTM G-150 (Critical Pitting Temperature)
ENVIRONMENTAL SIMULATION TESTS are carried conforming to ASTM B-117 & G-85. These tests are basically accelerated tests to determine how a particular material or a coating can behave in a specific environment. This test is most useful for relative comparison of two materials/ coatings subject to same environment.
ELECTRO-CHEMICAL TESTS are performed based on the electrode kinetics which take place as a result of the corrosion process. These are then interpreted to study both the corrosion rates and qualitative behavior of Corrosion mechanisms.
Some of the common electro-chemical techniques used at AEIS and their significance is given below:
- Potentiodynamic Polarization methods are used for conducting cyclic polarization to determine localized corrosion phenomena like Pitting potential, Critical Pitting, temperature, Crevice Corrosion, passivation onset etc. Some advanced techniques like Electro-chemical potentiodyanamic re-activation are used study the Sensitization in Stainless Steels.
- Linear polarization Resistance (LPR) is commonly used method to determine Corrosion rate from icorr (Corrosion current) by using Stearn & Geary equation and is computes by finding the Tafel Slopes of the Anodic Polarization Curves.
- Electro-Chemical Impedance spectroscopy (EIS) uses a small amplitude sinusoidal excitation signal to the system under investigation and measure the response (current or voltage). This method is used to study the behavior of Non-conducting Coatings and perform lifetime prediction of these coatings.
- Electrochemical Noise(EN) uses the fluctuations in the Current or Potential of corroding component to determine the onset of localized Corrosion phenomena like Pitting , SCC and Crevice Corrosion.
FIELD TESTS at AEIS can fall into the following categories
- On-line monitoring which uses different probes and electrochemical methods to determine the behavior of a structure or components during service
- Off-line Monitoring includes destructive testing like analysis of Corrosion products, NDT methods like UT thickness measurement to predict remaining wall thickness and determining the presence of localized corrosion using EN technique.
- Coating Inspection for detection of holidays in coatings, determining if the coating thickness is as per specification or using EIS (Electrochemical Impedance Spectroscopy) to predict the lifetime of a coating.
