Corrosion is a metaphysical certainty. The oxidation process happens to virtually all metallic equipment, structures and surfaces and is a response to its surroundings. The only question is when it actually manifests itself. Numerous factors play a role in a metal's corrosion rate – such as ambient temperature, environmental conditions or level of moisture in the air – but like it or not, it's bound to occur at one point or another.
The way in which to assess for this natural occurrence is through corrosion testing. As its description suggests, corrosion testing is a scientific method that examines metal's resistance to corrosion. There are several ways to test for corrosion, but in essence, the process uses substances or agents that are designed to induce corrosion and measure how quickly a metal or alloy breaks down due to the resulting chemical reactions. These types of tests are invaluable in the wide variety of industries in which metal is used by providing professionals with insight on the quality of the metal materials they're using and how quickly it may degrade. Protective measures, such as sealants or coatings, may need to be applied to improve overall performance. Polymers and coatings can also be used to test for corrosion, as these agents are designed to resist corrosion or, at the very least, slow its development.
Just as there are several types of metals, there are a number of different corrosion test methods. The following are the most common and dictate how corrosion tests are performed:
Salt spray testing
The gold standard of corrosion assessment, salt spray testing is perhaps the most common test solutions of them all, largely due to its effectiveness, scalability, repeatability, speed and cost.
There are also several salt spray testing methods. The most well known is referred to as ASTM B117. Here, a 5% concentration spray is applied to the metal in an enclosed chamber for a given period of time. There is almost no limit to how long this process can last; the observation period can be as little as a few hours or last up to six months or longer. The fact that this all occurs within the confines of an enclosed chamber enhances the corrosive environment as the pressurized air used to apply the solution produces a salt water fog or mist that saturates the air.
"There are also several salt spray testing methods, the most well known is ASTM B117."
Another salt spray testing option is called ASTM G85. This one is very similar to B117, but may be used as an alternative when the tested metal is likely to be installed or used in a highly corrosive environment. Replicating these more aggressively harsh conditions can be done a number of ways, like lowering the pH level of the testing solution – such as by using acetic acid, diluted electrolytes or cyclic acidified salt spray – or modifying the atmosphere.
Deciding on which salt spray test to use largely depends on what's being examined. ASTM B117 is frequently selected to assess passivation of stainless steels and to determine the rate of mass loss for materials where there's more than one metal. In other words, when you're comparing two or more metals to see which would break down faster. ASTM G85 may be a preferable method for testing coatings, paints or sealants that are designed to enhance corrosion resistance.
Standardized high humidity tests
At its essence, humidity is atmospheric moisture that derives from water vapor. Standardized high humidity tests don't require the use of any sprays, but do typically use enclosed chambers to enhance moisture content, which incidentally is the most common contributor to aircraft corrosion. High humidity tests are designed to assess the overall effectiveness of coatings used on aircraft or any structure aimed at slowing the rate at which metal deteriorates.
"ASTM D2803 tests specifically for filiform corrosion."
Like salt spray, there are different high humidity testing options, but the two most common are ASTM D2803 and ASTM D2247. The former tests specifically for filiform corrosion, which is a type of crevice corrosion. Bolt heads, washers and gaskets are among the materials that are most likely to experience crevice corrosion, as they're typically used in areas where oxygen flow is limited. When filiform corrosion occurs, it does so under thin organic coatings where the materials selection is a metal substrate.
The latter – ASTM D2247 – is another option and usually examines coatings. Yet instead of examining for a specific type of corrosion, this one assesses how effective a coating is in reducing the corrosive impact of water, a chief contributor to the chemical breakdown of metals. The goal here is to fill the chamber with enough compressed air so condensation forms on the coated metal being evaluated to see how it holds up under wet conditions.
Alternate immersion testing
When a metal, coating, alloy or polymer is in outdoor environments, it's a given that it will be wet on some days, dry on others. That is the crux of alternate immersion testing, which involves rotating a specimen in drenching-wet and bone-dry environments. When metal is fully immersed in water – to the point of being soaked – it induces and exacerbates cracking. This is a type of stress corrosion. Since cracking is progressive – it gets worse over time – stress corrosion testing is important because it can better determine if cracking will become so significant as to render a metal useless or structurally unsound. Alternate immersion testing is often used in industries where metal is used for things like bridges, roads or other forms of construction. The term "stress" can refer to both tensile stress, meaning weight-bearing, as well as oxidation.
Much like standardized high humidity tests and salt spray testing, there are two primary types of alternate immersion testing, only these are known as ASTM G44 and ASTM G47. ASTM G44 is leveraged for the testing of aluminum and ferrous alloys and immersing the test subject in a 3.5% sodium chloride solution. Synthetic seawater is another solution that is used for ASTM G44.
ASTM G47 is mainly used for testing specific aluminum alloys. The duration of this version of alternate immersion testing typically lasts longer than ASTM G44, anywhere from a couple of weeks to as long as a year.
An offshoot of alternate immersion testing is known as cyclic corrosion testing, or CCT. Frequently done within the automotive industry, cyclic corrosion testing is designed to replicate some of the conditions that are common in outdoor environments, in which the environment is changeable (e.g. snow, drizzle, pouring rain, etc.). Some of the environments used in CCT procedures include ambient (laboratory), fog (chamber) and dry-off. This latter one can be done in a laboratory or a chamber, so long as there is enough air present to allow for the test specimen to dry naturally. Virtually any metallic substance or component that has the potential to be exposed to moist conditions can be used for cyclic corrosion testing.
How is corrosion measured?
Although it's not always immediately identifiable, corrosion is distinctive, in that it can be recognized by color, composition and spread. Just as there are several types of corrosion – such as pitting, crevice, intergranular and others – there are gradations of corrosion, which ultimately determine the corrosion rate. Calculating this rate requires several measurable pieces of information, including density of the metal, weight or mass loss, surface area, and time – meaning how long a metal was exposed to a corrosive environment.
These specifics – weight loss, density, area and time – can be plugged into a formula that results in a corrosion rate, which is measured in mils per year, or MPY. The metric equivalent of MPY is MM/Y, or millimeters per year. Several website offer corrosion rate calculators.
Corrosion rate is an extremely important data item because it can help developers, manufacturers, factory workers and other industry professionals evaluate a substance's lifespan. For example, if a corrosion rate is high for a particular metal or alloy, knowing this can help determine whether a substitute material is preferable. It also helps determine the type of maintenance or treatments that may be required to diminish or mitigate the impact of corrosion.
Why choose Auto Technology?
Whether your industry is aviation, automotive, energy or virtually any other that uses metal or coatings, only the very best of materials will do in a performance-driven economy. Auto Technology offers the technology, proficiency and experience to test for corrosion in a variety of environments. We do this by putting your equipment through a battery of testing services that can evaluate susceptibility to the harsh effects of corrosion and specific types, such as filiform, intergranular, pitting, galvanic and crevice. We're also equipped to stress test your products under a variety of conditions, from temperatures as low as 70 degrees Celsius to as high as 180 degrees Celsius. We also offer dozens of test that run almost literally from A to Z, including ASTM B117 to JASO M610 to UL1703. Auto Technology's certified capabilities are also wide-ranging, from acetic acid salt spray to UV exposure and water immersion.
For truly accurate corrosion testing done right, contact us today.