The gradual degradation of materials as a result of their interaction of with their surroundings, also known as corrosion, has a tremendous impact in science and engineering. In some cases, like metallic structures, unmanaged corrosion can lead to catastrophic and life-threatening events. For this reason, understanding the corrosion rate of materials is of utmost importance. In this post, we explain in detail the meaning of this physical parameter, its units, how it’s measured and how we can use electrochemistry to quantify it.
What is the meaning of the corrosion rate?
The corrosion rate is a number used to describe how metals degrade in specific conditions such as in acidic or salted solutions.
This number is very useful to understand how each metal will interact in real-life situations and helps engineers and scientists with their material choices, as well as deciding whether a protective coating is needed.
Two real-world applications where the corrosion rate is important is in mechanical structures and the food industry.
For example, when designing a structure for a ship that will sail through the sea, it is important to understand how the metals used in the ship’s hull will react to the high salinity of the ocean. Otherwise, the hull would slowly degrade putting the passenger’s and crew member’s life at risk.
Similarly, tetrabrik and yogurt packaging often have thin aluminium layers to prevent oxygen from getting in touch with the food, thus extending the shelf-life of the products. This aluminium layer is coated by a thin polymer coating to prevent the aluminium from corroding and contaminating the food. Therefore, understanding how this protective coating affects the corrosion rate of aluminium is essential to ensure food safety.
Corrosion rate units
There are several ways to represent the corrosion rate. Some of the most common are:
- microAmpere per cm2 : µAcm-2
- milli-inch per year: mpy
- micrometer per year: µmy-1
- millimeter per year: mmy-1
- gram per m2 per day: gmd
- milligram per dm2 per day: mdd
From all these units, perhaps the most common one is mmy-1 , since it provides an indication of the thickness of the metal corroded during a specific timeframe. This what we call units of penetration and are useful from an engineering standpoint, since they are useful for the engineering design process.
How to measure corrosion rate
There are mainly 2 ways to measure the corrosion rate:
- measuring the weight change of the metal after a specific time of exposure to the target environment
- using electrochemistry.
While the first option is accurate and believed to be the best type of corrosion test, it is a very lengthy process. Corrosion itself is a slow reaction and weight based measurements might take weeks, months or even years before the experiments are finished.
Electrochemistry based methods, on the other hand, are able to measure corrosionaccurately within a matter of hours or even minutes. This is because electrochemical methods are able to actively interact with the test sample to accelerate the corrosion process, enabling measurements of very low corrosion rates.
Corrosion rate equation
The corrosion rate formula is quite straightforward:
However, determining the amount of mass dissolved at any given time using traditional methods requires a very long time. Luckily, with electrochemistry we can estimate the mass using Faraday’s Law of Electrolysis, which converts the corrosion rate equation as follows:
where:
i is the corrosion current, which can be obtained from the Tafel equation, the Butler-Volmer equation or the Stern-Geary equation,
M is the atomic mass in g/mol
n is the number of electrons of the corrosion reaction
and F is Faraday’s constant 96485 C/mol
This equation so far allows us to determine the corrosion rate in g/s from results obtained from electrochemical experiments. To transform it into penetration units, it must be divided by the material’s density and the surface area exposed:
where:
ρ is the density of the material being corroded in g/cm3
A is the exposed area in cm2
To transform the resulting corrosion rate to milli-inch per year (mpy) or millimiter per year (mmpy) units, it just needs to be multiplied by 0.13 for mpy conversion and by 0.00327 for mmpy conversion.
Steps to calculate corrosion rate
Estimating the corrosion rate of a material using electrochemistry can be done in just a few steps:
- First of all, measure the open circuit potential (OCP) and allow its value to stabilize. Depending on the material this may take a few minutes up to a few hours.
- Perform a linear sweep from -100 mV with respect to OCP to 100 mV above the OCP. This swee must be done at a very slow scanrate < 1 mV/s. This ensures that the measurement is performed under steady-state conditions by giving the system enough time to stabilize at each applied potential.
- With the obtained i-V data, construct a Tafel plot to extract the corrosion current. For best results perform a fit using the Tafel or Butler-Volmer equations.
- Convert the corrosion current into a corrosion rate using the corrosion rate equation specified above.
As we have seen, quantifying corrosion is extremely useful for the design of everyday products. While the best way to characterize the corrosion is with lengthy tests, which can last up to years, that involve weighing metal pieces before and after corrosion, this is certainly not practical. Luckily, we can use electrochemical measurements to get accurate corrosion rates within a few minutes or hours, depending on how fast the OCP for the specific material takes to stabilize.
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