The Vital Role of Corrosion Testing in Developing High-Performance Alloys

Emerging Technology

Authors: K Stritch

Carpenter Technology relies on extensive, in-house corrosion testing capabilities to ensure our alloys can withstand a variety of different service environments. Whether immersion testing in a specific chemical solution, salt spray testing, stress corrosion cracking susceptibility testing, or electrochemical testing, our R&D team has the testing expertise to address any corrosion-related issues.

Immersion Testing

Immersion testing in various chemical solutions designed to mimic specific corrosive environments is a common and simple method to evaluate corrosion susceptibility of an alloy. During immersion testing, a sample of material is weighed and exposed to a solution of a certain concentration and temperature for a defined time, usually 24–72 hours. After exposure is complete, the sample is weighed again, and the weight loss is used to determine the degree of corrosive attack.

Over the years, ASTM/ISO standard tests have been developed around immersion testing to screen for specific kinds of corrosion-related failure mechanisms, such as pitting or intergranular attack. These tests are often used as quality assurance tests on production material but can also be useful in determining if an alloy is appropriate for use in a specific corrosive environment.

Salt Spray Testing

Steel alloys are often prone to oxidation (rust) when exposed to chloride-containing environments, such as seawater and humid environments. Salt spray testing is used to evaluate an alloy’s susceptibility to oxidation in these environments. Test samples are polished, cleaned, and often passivated to exposure to a salt-containing spray at a certain temperature. A testing period typically lasts between 200–1000 hours and is usually performed using a 5wt% NaCl water solution at 95°F, in accordance with ASTM B117.

During testing, the time of first rust detection is noted. After the exposure period, the samples are rated on a 1–9 scale based on the percentage of rust covering the sample surface. A rating of 1 corresponds to no rust present after testing, and a rating of 9 equates to > 80% rust on the surface. The time to first rust and final rating are then used to determine the alloy’s susceptibility to oxidation.

Stress Corrosion Cracking Susceptibility Tests

Stress corrosion cracking (SCC) is a common root cause of severe, sometimes catastrophic, failures of in-service components. While the exact mechanism of SCC is still a topic of academic debate, it is generally agreed that three criteria need to be satisfied for SCC to occur: exposure to a corrosive environment, a tensile loading stress, and a susceptible material microstructure. Over the years, it has been found that certain alloys are more susceptible to SCC than others. The two main test methods that Carpenter Technology uses to evaluate SCC susceptibility are slow strain rate testing (SSRT) and rising step load (RSL) testing.

SSRT is essentially a modified version of a mechanical tensile test that allows a tension sample to be exposed to a corrosive environment and elevated temperatures during testing. The test is commonly performed using a chloride-containing solution, but SCC can be evaluated in other corrosive media as well. A sample is first tested in a neutral environment, often just air, and pulled to failure using an extremely slow strain rate. The time-to-failure and ductility of the sample are measured after the test is complete. The test is then repeated on a sample that is exposed to a corrosive environment. The material is considered susceptible to SCC if the time-to-failure and/or ductility of the exposed sample falls below a certain threshold when compared to the results of the neutral sample. Susceptibility to SCC can be ranked across several different alloy grades tested in a certain corrosive environment by comparing the ratios of time-to-failure and ductility between the neutral and exposed tests.

Like SSRT, RSL testing is a hybrid of mechanical and corrosion testing. However, RSL testing is more focused on fracture mechanics and crack propagation while a sample is exposed to a corrosive environment. As a result, RSL testing can provide important material property and critical stress intensity fracture parameters, such as KISCC.

Electrochemical Testing

Electrochemical testing is an accelerated corrosion test that can provide important corrosion characteristic data covering a wide range of metallic corrosion, including, but not limited to, general corrosion, pitting/crevice corrosion, and galvanic corrosion.

The basic principle behind electrochemical testing is that a metallic sample is submerged in an aqueous, conductive solution. An electric potential is applied to the sample, allowing a flow of electrical changes to occur within the test cell. During testing, cathodic (positive) and anodic (negative) regions form on the surface of the sample, leading to electrochemical corrosion. Various electrochemical measurement and analysis techniques can then be used to quantify various kinds of metallic corrosion.

The corrosion lab at Carpenter Technology routinely runs electrochemical tests in accordance with ASTM G5, G61, and G150 that can provide important electrochemical data for any alloy in any aqueous solution. This data includes, but is not limited to, the critical pitting temperature (CPT) and critical pitting potential (CPP) and can detect if a re-passivation (sometimes referred to as protection) potential exists for a particular alloy per a given corrosive solution.

Research and Development Corrosion Testing

Carpenter Technology has a long history of developing new, corrosion-resistant alloys for use in applications spanning several commercial markets. Having the necessary corrosion testing capabilities at our main research and development facility has been critical to the success of our developmental efforts. Oftentimes, ASTM/ISO standard corrosion tests can be used as appropriate success criterion when aiming for a specific corrosion performance. However, certain situations require the need to develop modified and/or unique testing methods to properly assess the corrosion performance of an alloy targeted for a specific application. Carpenter Technology’s team of metallurgist and corrosion experts are ready to partner with you to provide solutions for any corrosion-related challenge.

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