Corrosion testing is one of a corrosion engineer's most important responsibilities. In fact, without corrosion assessment, mitigating or eliminating corrosion in any industry is almost impossible.
There are several reasons for corrosion examination. Sometimes, in a materials selection process for an industrial application, an evaluation of different kinds of materials in a specific environment is required. The assessment of a new type of alloy in different types of environments to compare with conventional commercial alloys; an estimation of an inhibitors' efficiency in reducing the corrosion rate of metals; and understanding the mechanism of corrosion are the other reasons.
Corrosion tests are usually divided into two main categories: laboratory tests and field tests, each of which has its merits and demerits. For example, the environmental conditions present in real-world applications are different to those in laboratory situations. Therefore, it is difficult to extrapolate the results of laboratory tests to industry settings. On the other hand, in laboratory tests, it is possible to accelerate the corrosivity of the environment to obtain results more rapidly, something that is impossible in field testing.
Here we'll take a look at the eight key corrosion tests that corrosion engineers use to help learn about and mitigate corrosion problems in the field.
Laboratory Corrosion Tests
One of the most common and simplest methods in laboratory tests is the immersion test. In this kind of test, whose procedure is clarified by ASTM and NACE, the weights of dried test specimens are measured by an analytical balance before and after being exposed to a corrosive environment for a specific period of time. Before and after samples are weighed, specific preparation should be carried out to remove any corrosion product or organic contaminants. The corrosion resistance of the samples is generally calculated as the corrosion rate in terms of weight loss or thickness loss in mils (0.001 inch) per year (mpy) or millimeters per year (mm/yr). The results depend on the type of metal (specific weight) being tested, the exposed surface area, and test duration factors.
Some visual examination is also suggested to evaluate localized corrosion like pitting or exfoliation. Furthermore, optical or scanning electron microscopes; elemental and compositional analyses such as energy dispersive X-ray spectroscopy (EDX); X-ray diffraction (XRD); and energy dispersive X-ray spectroscopy (XPS) are useful techniques to evaluate the corroded surface and corrosion product more precisely.
There are several ways to evaluate the pitting corrosion of tested samples. Determining the density of pits (number of pits in a specific surface area) or pitting factor (ratio of depth of deepest pit divided by the value of thickness loss due to uniform corrosion) are two important methods to evaluate pitting corrosion. There are different types of practical tools to measure pit depth. A contour gauge can be used to achieve a profile of pit depth when it is impossible to use pit gages.
Salt Spray / Fog Testing
Some test samples and procedures are designed to assess specific kinds of corrosion, such as crevice corrosion, stress-corrosion cracking, and erosion corrosion. The atmospheric corrosion of coated samples can be examined by salt spray or fog testing. Here, a 5% NaCl solution is atomized in a chamber at a temperature adjusted to 95°F (35°C). The time that samples can resist against corrosion is the criterion used to understand test sample durability. Although the environment in a salt spray test is a kind of accelerated marine atmosphere, it is accepted that the salt spray results could extrapolate to other atmospheric environments. (For more on corrosion in a marine environment, see Marine Structures Create Unique Challenges for Third-Party Inspectors.)
The Weathering Test
In another atmospheric method called the weathering test, the durability of organic, paint-coated samples is examined by exposing them to UV light and cyclic cooling-heating along with a corrosive environment.
Electrochemical tests are the other category of laboratory tests that can provide valuable information about corrosion electrochemical reactions and the mechanisms behind them. A potentiostat instrument is usually used to perform this sort of test. A three-electrode setup, including working electrode, reference electrode, and counter (auxiliary) electrode, are usually used. Potential, current, and time are three important parameters in electrochemical tests. In these tests, an applied potential generally scans in a certain range and the current is measured.
There are various types of electrochemical corrosion tests. Each type is used for a particular purpose.
- Linear Polarization Resistance (LPR): The simplest electrochemical corrosion test is linear polarization resistance, in which current is measured when the applied potential scans in a narrow range (~20 mV) from lower to higher than corrosion potential (Ecorr). The slope of the current versus the potential curve shows the polarization resistance, which is inversely related to the corrosion rate. This test is very fast and straightforward, and is usually accepted as a kind of non-destructive test. Moreover, this method is very useful to measure extremely low corrosion rates. This is important in some industrial systems such as food processing, nuclear, and pharmaceutical equipment.
This image shows the LPR curve. The slope of the line shows polarization resistance (Rp).
- Potentiodynamic Polarization Tests: The passivation behavior of active-passive metals, like stainless steels, can be evaluated by potentiodynamic polarization tests. In this method, the potential scans in a wide range. Critical current density, passive potential, and passive current density can be extracted from this test.
- Cyclic Polarization Method: This is the other kind of test that is used to determine the tendency of active-passive metals to localized crevice or pitting corrosion. In this test, the sweeping direction of applied potential is reversed at some potential in the transpassive region. The intersection between forward and backward scans shows the tendency and intensity of localized corrosion.
This image shows the cyclic polarization curve, which is used to evaluate pitting corrosion. Less ER and a bigger metastable pitting loop shows more susceptibility to pitting corrosion.
- The Electrochemical Potentiodynamic Reactivation (EPR) Test: This is the other test that has been suggested to predict the tendency of stainless steels to sensitization or intergranular corrosion. Electrochemical potentiodynamic reactivation is very simple and fast in comparison to other conventional intergranular corrosion tests that are suggested by ASTM A-262, such as Huey or Streicher.
The abovementioned electrochemical tests are conducted under DC conditions. However, understanding the Helmholtz double layer (which can act as a capacitance) or adsorption of inhibitors on the metallic surface (which can act as an inductance) requires alternative currents. This type of test is called Electrochemical Impedance Spectroscopy (EIS), and can reveal valuable information about corrosion mechanisms. Moreover, this technique is very useful when the overall electrical resistance in the electrochemical system is very high, such as when a sample is covered by thick organic coatings or is immersed in organic solutions.
Field Corrosion Tests
Installing corrosion coupons is a very simple and common method for monitoring corrosion in pipelines, heat exchangers and storage tanks. The coupons are inserted into a plant or equipment with a coupon holder for a period of time. (Learn more about coupon holders in How Corrosion Coupon Holders Can Preserve Your Valuable Assets.) Although many factors can influence the location of coupon installation, the coupons are usually placed in locations where severe corrosion is expected. The change in weight and size or visual inspection will be considered after retrieving the coupons. The drawback to this method is that it is impossible to accelerate the environmental conditions to achieve faster results.
Ultrasonic Thickness Monitoring
An ultrasonic thickness (UT) gauge is one of the instruments used to monitor the internal corrosion of pipelines or storage tanks. The ultrasonic sound wave, which is produced by an ultrasonic transducer, traverses to the back wall and reflects back to the source, making it possible to calculate the thickness of metal by measuring the reflection time and considering the velocity of sound waves in the tested material. The UT gauge test is useful when there is no access to both sides of a test specimen. (More information about ultrasonic testing can be found in CUI Detection Techniques for Process Pipelines (Part 2).)
Electrical Resistance Testing
Electrical resistance (ER) probes are used to measure the corrosion rate of coupons, especially when the on-line corrosion rate in required. When a metal corrodes in an environment, its electrical resistance will increase due to a reduction in the thickness or surface area of a cross-section. By measuring the change in electrical resistance of metal over time, the rate of metal dissolution can be determined and the corrosion rate can be calculated in mpy or mm/yr. The ER probes can be used in any kind of environment, including aqueous solutions, oil (hydrocarbons), soil, gas, and atmosphere. The probe can be produced in various geometries depending on the type of metals, system, and environment being tested.
There are many other tests that are used to monitor the structures protected against corrosion by cathodic protection. Most of these tests are based on the measurement of the electrochemical potential of structure versus environment.