B. Helmersson, Outokumpu, Avesta, Sweden
Pitting corrosion can seriously impact the service life of stainless steel components in hydrocarbon processing applications. The risk of this localized attack increases as the temperature increases. Therefore, measuring the critical pitting temperature (CPT) at which corrosion starts can help design engineers compare the likely performance of different stainless steel grades.
Unlike yield strength, the CPT is not an inherent material property. Its value can vary considerably depending on factors such as the test environment, methodology and how the specimens are prepared. However, the CPT is useful for ranking and comparing different materials’ resistance to pitting corrosion in a clearly defined environment. It can evaluate different variants of the same stainless steel grades, surface finishes, slight differences in chemical composition, heat treatment or any other material variations.
CPT testing shows what happens to the corrosion resistance when the material is changed. A higher CPT value will show that one stainless steel grade will perform better than a lower-value grade; however, the value cannot be used directly to predict performance in a real-world application.
What is pitting corrosion? Stainless steel gets corrosion resistance from a thin, invisible, insoluble layer of chromium, iron oxides and hydroxides that form spontaneously on the surface. This passive layer is only a few nanometers thick, but it effectively keeps the metal underneath isolated from its surroundings. This means the electrochemical reactions that cause corrosion are slowed to a fraction of what they would be without the passive layer. Moreover, it reforms spontaneously if the metal beneath the passive layer is exposed, such as by mechanical damage or scratches.
Some environments can cause the permanent breakdown of the passive layer, causing corrosion on the unprotected surface. In uniform corrosion, this breakdown is widespread. In contrast, pitting and crevice corrosion occurs through the localized breakdown of the passive film. In many practical situations, stainless steel corrosion failure occurs due to this localized attack.
Pitting corrosion is characterized by surface attack at small discrete areas, typically in the presence of chlorides, and can be associated with a local discontinuity of the passive film, such as an inclusion or surface damage. However, pits can also initiate on a seemingly perfect surface. A pitting attack often propagates at a high rate, quickly causing corrosion failure. Pits can appear small at the surface but may be much larger underneath. Moreover, the small pit opening at the surface may be covered by corrosion products. Therefore, pitting corrosion could remain undiscovered until it causes perforation and leakage.
No fixed size defines a pit. For the purposes of corrosion testing, pits shallower than 25 micrometers (µm) are often disregarded. A pit can be smaller, but it can become challenging to distinguish pits caused by corrosion from naturally occurring surface imperfections.
CPT testing according to the American Society for Testing and Materials (ASTM) G48 method E. The ASTM G48 standard is a well-known and frequently used method for testing stainless steels and related alloys. The standard describes six methods to perform pitting and crevice corrosion tests, all of which are constant-temperature immersion tests in ferric chloride solutions.
To measure the CPT of stainless steels, ASTM G48 method E is used (FIG. 1). This method involves testing samples over a range of temperatures to find the lowest point at which pitting corrosion occurs—the CPT. The sample is exposed for 24 hr in a chloride-rich, oxidizing and acidic solution of 6% ferric chloride with an addition of 1% hydrochloric acid to stabilize the pH. The tests are performed with new samples for each test temperature in increments of 5°C.
The advantages of this method are that the test setup is simple, and it is easy to test complex sample geometries. If it is of interest, the corrosion resistance of edges can be evaluated.
However, this method does have some disadvantages, especially regarding the long test durations and work intensity, since several samples must be prepared, cleaned and evaluated. Furthermore, while testing the corrosion resistance of edges can be advantageous under specific circumstances, the exposed edges, which are typically more prone to attack, can also be problematic. Edge attacks can influence results by causing galvanic effects that, to some extent, protect the surrounding surfaces from attack. This method cannot be used on either low-alloyed or very highly-alloyed grades.
CPT testing according to ASTM G150. An excellent alternative test method is ASTM G150, which uses electrochemical measurements to determine the CPT. Combined with a flushed port cell, or Avesta cell, this method eliminates the influence of edge attack by only testing the sample surface. In the Avesta cell, the edges of the sample are flushed with distilled water to prevent unwanted crevice corrosion from interfering with the test (FIG. 2).
The specimen is exposed to one molar sodium chloride (NaCl) solution at 0°C and held at a constant potential of > 700 millivolts vs. saturated calomel reference electrode. The test solution is then heated with a ramp rate of 1°C/min. The current density is recorded as the temperature rises.
A current spike occurs when corrosion starts, and the CPT is the temperature at which the current density exceeds 100 µA/cm2. In the case shown in FIG. 3, the CPT is around 54°C. Pitting on the test specimen and the absence of crevice attack are confirmed visually after the test.
The major advantage of the ASTM G150 method is that it is quick; it takes only 2 hr–3 hr, including specimen preparation and setup, depending on the steel grade. It also provides precise results. Only the surface is tested; unlike the previous method, no harsh or unsafe chemicals are used.
The key disadvantage of the method is that it requires more complex electrochemical test equipment. It is also more difficult to test non-flat products, although this can be done with some ingenuity. Finally, like ASTM G48 E, it cannot be used on low-alloyed or highly-alloyed grades, but the range is more restricted in this case.
Comparing the results of the two test methods. As seen in FIG. 4, the ASTM G48 E method tends to be more aggressive regarding the size of the pits produced and corroding the sample edges. In contrast, ASTM G150 causes an attack on a limited flat surface.
The difference in the CPT results of the two test methods is also shown in FIG. 5. While the hierarchy of grades is normally the same, the actual CPT figures obtained can vary considerably. For example, for 254 SMO (a very high-end austenitic stainless steel), the ASTM G48 E figure is 65°C, while for ASTM G150 it is 85°C. However, the difference may be partly attributed to differences in surface preparation.
Different products and surface finishes, such as mill finishes, may show CPT values that differ from those shown in FIG. 5. That is why, to rank steel grades, the surface is ground before testing to try and minimize these effects.
An important point to note is that a certain CPT is only relevant to the specific type of test method used. CPTs obtained using different test methods must not be used to compare the corrosion resistance of materials.
CPTs: A useful relative measure. Understanding pitting corrosion and how it is influenced by environmental factors, such as temperature, is crucial to ensuring the safety and reliability of stainless steel components in chloride-rich environments. CPT testing provides a useful way of comparing the susceptibility of different grades. However, it is always important to remember that the actual CPT figure can vary depending on the test method and other factors, such as mill finish and specimen geometry. This is why a CPT is always a relative measure rather than an absolute property. HP
BJÖRN HELMERSSON is a corrosion specialist at Outokumpu’s R&D facilities in Avesta, Sweden.