Reinforced concrete structures often suffer from chloride-induced corrosion. The most commonly used reinforcement material is carbon steel (CS), which has a lower corrosion resistance than stainless steel (SS). Because of this, SS is recommended as a solution to this issue. SS differ in terms of corrosion resistance, however, and the need to determine their corrosion resistance is essential. A standard test method that is reliable, reproducible, and quick is crucial.
Corrosion on the reinforcement of concrete structures frequently occurs due to chloride exposure from environmental factors, such as exposure to de-icing salts or a marine atmosphere. When chloride reaches the reinforcement, corrosion begins to transpire. Corrosion leads to infrastructure that fails to meet its designated service life and requires maintenance and/or repair in spite of compliance with prescriptive durability design requirements.
Typically, dense concrete with low porosity is utilized to protect against chloride ingress. On the other hand, the denser the concrete, the less workability is available during the concreting process. Additionally, increasing concrete cover depths is commonly used as a solution, but this can result in concrete cracking.
Unfortunately, a service life of 50 years or more cannot be guaranteed when there are surface chloride concentrations in excess of 3.5 wt%/lb. In cases where optimization of the concrete composition comes to a limit, there is no alternative but to alter the reinforcement material (i.e., replace CS with SS). In such conditions, doing so will accomplish the desired service life.
In the CORROSION 2021 conference paper on which this article was based, Sylvia Kessler (Helmut-Schmidt-University/University of the Federal Armed Forces Hamburg, Hamburg, Germany) notes that the corrosion initiation phase can take years and it depends on the surface chloride concentration in contact with concrete, the chloride penetration resistance of the concrete, concrete cover depth, and the chloride threshold value of the reinforcement.1
The chromium used in SS elicits an inert oxide layer on the metal that strengthens the passivation. For this reason, SS are more corrosion resistant than CS. Additionally, SS contain elements like nickel and molybdenum, which promote corrosion resistance. SS can differ in composition through the use of numerous alloying elements, allowing them to be tailored to the appropriate application.
It is crucial to grade the corrosion resistance of SS; however, there exists a lack of methods for doing so. Only one “crude” process is used—the pitting resistance equivalent number. It does not consider how SS react to alkaline environments, nor does it consider the microstructures that affect the corrosion behavior. For these reasons, a proper test method is needed to evaluate SS corrosion resistance.
As far as corrosion testing goes, tests that use long-term exposure elicit the most dependable results for gauging the corrosion resistance of metal. The goal of such tests should be to mimic natural conditions as closely as possible, while also being fast. It is important to note that the quicker the test, the further the results can differ from natural conditions. Standardized tests (multiple, run in parallel) with set conditions and procedures that lead to comparable results are essential.
Much literature exists containing different approaches to determining corrosion resistance of SS; however, not all contain the necessary critical information. Table 1 presents the nomenclature of SS used in literature.
Because researchers are using different tests, as seen in literature, it is necessary to come to a consensus on standard parameters. Some factors commonly tested include different methods, pre-conditions, measurements, and evaluations.
In certain studies, even after extensive exposure, the SS never began to corrode. The main focus in corrosion resistance testing should be on the initiation phase—the key parameter being the chloride threshold value, which estimates the time to corrosion. Knowing this information enables us to estimate the service life of the infrastructure. Other factors, which are part of the propagation phase, are important, but not vital, such as pitting potentials, corrosion rates, mass loss, etc.
There is no standard procedure for evaluating the chloride threshold value of CS or SS. Knowing this information would provide the ability to compare the corrosion resistance of metal reinforcement. As it stands, we know that different grades of SS have different chloride threshold levels for corrosion initiation; however, the variation in test methods used led to scatter, invalidating the data. The different test methods are presented in Table 2, along with the pros and cons for each method.
A compliance test, recommended by L. Bertolini, called the ECISS test, enables evaluation of the cumulative density function of the chloride threshold for corrosion initiation and includes different grades of SS. To determine whether corrosion initiation occurs, the test stipulates that the maximum current density does not exceed 80 mA × m-2 and the steel does not show any visual signs of corrosion.
The ECISS test requires that the procedure be repeated with different chlorides mixed in each time until nine out of 10 samples pass. Minimal equipment is required—only a potentiostat and a current data logger system are needed to perform the tests. Nor are high advanced electrochemical measurements necessary. ECISS evaluation methods are straightforward and allow for an estimation of service life.
In conclusion, the best determiner of corrosion resistance is the chloride threshold value. Of the different tests presented, only one establishes this value in a reliable, reproducible, and fast manner—the ECISS test. This test can allow for the proper selection of SS grade given durability requirements and cost constraints.
Kessler emphasizes that additional tests are necessary to describe corrosion resistance, such as instances of welded steel, cracked or carbonated concrete, etc.
1 S. Kessler, “The Determination of the Chloride Threshold of Stainless Steel in Concrete – a Review,” CORROSION 2021, paper no. 16179 (Houston, TX: NACE International, 2021).