How to Test Metal for Rust

What dust is to furniture, rust is to metal: It's unsightly, naturally occurring and has a tendency to spread just as soon as it is detected. While neither can be stifled entirely, you can forecast the likelihood of rust development through various corrosion testing techniques.

What is corrosion testing?
At its essence, a corrosion test is an observational exercise designed to gauge the speed at which metal rusts by accelerating the rusting process through a series of corrosivity testing methods. The most common type is called salt spray testing. Salt – or more specifically, sodium chloride – is a natural enemy to metal and will cause it to corrode over time based on how much is used and the surrounding conditions. An environmental test chamber serves as the "arena" for these experiments. The test metal is placed inside and users can artificially simulate the desired conditions, including temperature, relative humidity, wetness, among a few other controllable aspects to the space. A nozzle is used to spray the test metal at regularly occurring intervals over time, which could be days, weeks or even months.

"The purpose of corrosion testing is to see just how resistant a metal is to rusting."

The purpose of corrosion testing is to see just how resistant a metal is to rusting and what applications, treatments or coatings can be used to diminish or frustrate its development. Numerous industries take advantage of simple corrosion test methods so users can get the most out of their equipment, parts and tools, thereby helping to lengthen their lifespan. They include automakers, manufacturers, jetline producers, the U.S. military, defense contractors and many more.

What is the difference between corrosion and rust?
Because the effects of rust and corrosion look so much alike, these two terms are often used interchangeably. While they do have a lot in common, they actually have more contrasts than similarities. Perhaps the most salient one of them all is where they exist, meaning the types of material. Whereas corrosion can occur on multiple surfaces above and beyond metal – such as wood, polymers and ceramics – rust targets iron and alloys of iron, such as steel, nickel, chromium and celestrium. The two are also unique at the molecular level. Corrosion manifests itself when materials are exposed to a combination of contributing factors, such as chemicals, temperature and wind. Rust is a step along the corrosion process. In other words, rust is a manifestation of corrosion rather than its own  independent phenomenon and can be identified by its color and appearance (usually an orangey brown).

Understanding these subtle differences can help determine the best type of testing to implement and the speed at which corrosion is taking place.

What speeds up rusting?
Yet another distinctive aspect to corrosion and rusting is what these two processes produce, which speak to the circumstances that influence their occurrence. When chemicals are present – such as hydrofluoric acid, chromic acid and sulfuric acid – in addition to air – corrosion produces oxides. What these oxides are made up of depends on the circumstances. Rust, on the other hand, only results in iron oxide and is produced only when air and moisture are introduced. Unlike corrosion, chemicals don't have as significant an influence on rust development.

Thus, atmospheric conditions such as air, humidity and wetness tend to have the largest impact on how to make iron rust. The age of the metal plays a role as well. Generally speaking, the older the iron or iron alloy happens to be, the more vulnerable it is rust. 

What metal rusts the fastest?
This is a question that many people ask, but given the differences between rusting and corrosion, often does not produce the answer many people expect. As previously referenced, technically the only thing that produces rust are materials made from iron. As a result, steel is almost by definition the metal that rusts the quickest.

Perhaps a better question is what metal corrodes the fastest. But even here, there is no universally accepted answer in the scientific community. Given that there are so many contributing factors that play a role – environment, time of exposure, wetness, air temperature, humidity etc. – it's difficult to pinpoint just one.

"Precious metals generally do not corrode."

However, while corrosion affects most metals, there are several in which corrosion won't occur, regardless of the environment and contributing factors introduced. These chiefly include precious metals, namely gold, silver, platinum and palladium. Many metals are corrosion-resistant but nonetheless will corrode over time, like brass, aluminum, stainless steel and copper. 

How is rust tested?
When it comes to precisely how rust is tested, you need to have the contributing factors and environments that cause it in the first place. Chief among these is oxygen, water and iron, or the metal itself. In the absence of even one of these, rust won't happen. It's the manner in which each is introduced – and to what degree – that ultimately dictates when and how quickly something will rust. It's the addition or contribution of other substances that can help to mitigate it.

For example, calcium chloride is an element that naturally makes air less moist. This would enable an ironclad nail, screw or other piece of machinery to be more resistant to rusting.

But if that same test subject were dropped in a salt water solution in isolation, severe rusting would occur because salt water conducts the electrolytes that produce the rust-causing ions.  A special tool, called a ferroxyl indicator, helps to show exactly when rusting takes place so it can be observed in real time.

Specimen preparation, racking, timing, calibration of the environmental test chamber and the type of test conducted all have an impact on how rust is tested.

How do you test for pitting corrosion?
What also influences which test to conduct is the type of corrosion you're testing for. And among the most destructive is called pitting corrosion. Unlike uniform corrosion, which typically spreads evenly across a surface material, pitting corrosion is more localized, targeting a specific area by forming "pits," hence the name. Because it can form in only one area, this can make it harder to spot. These pits bore holes into the surface that leads to cracking, which can take on a number of different shapes and directions, including elliptical, vertical, horizontal or shallow. Generally speaking, though, pitting corrosion forms in a more vertical direction. Bridges, roads, aircraft and iron alloys used in infrastructure are often vulnerable to this type of corrosion, making it important to test for it to enhance safety.

"Rust is an ongoing concern for state governments and road crews that must maintain roadways."

Rust is an ongoing concern for state governments and road crews that must actively maintain the physical strength of their roadways, which are in varying stages of disrepair due to overuse. According to the American Society of Civil Engineers, nearly 40% of the nations' bridges are 50 years of age or older. Additionally, several states throughout the U.S. have thousands of roads that are deemed "structurally deficient."

The steel used in bridge construction and development are ideal candidates for pitting corrosion testing so crews and installers can gauge what treatments are necessary to enhance their lifespan. 

The most common test for pitting corrosion is called ASTM G48. There are several branches, or versions, of the ASTM G48 test, including A, C and E. The difference between each include the temperature of the environment and how long the subject is exposed to the requisite treatment. An ASTM G48 A, for example, uses a ferrite chloride solution on the specimen with an ambient temperature ranging up to 60 degrees Celsius. The test itself is usually done over a 24-hour period but may be for as long as 72 hours.

The goal is to see at what temperature pitting corrosion will become apparent. The answer dictates the appropriate solution.

How do you prevent and protect against pitting corrosion?
While certain chemical compounds – sodium chloride especially – contribute to pitting corrosion, others have a deterring impact. Some of the most effective include chromium, nitrogen and molybdenum. These elements can be added to paints, sealants and coating treatments that enhance steel's resistance or to the steel itself during composition. Determining pitting resilience – which the industry refers to as pitting resistance equivalent – can be calculated with the following formula:

% of Chromium + 3.3 x % of Molybdenum + 16 x % of Nitrogen = PRE

The higher the number resulting from this formula, the more resistant the test subject is to pitting corrosion. 

How do you treat pitting corrosion?
Pitting corrosion is not only dangerous when present, but it can be very difficult to guard against since its formation tends to be unpredictable. Aside from including the aforementioned elements in coatings or the steel itself, using cathodic and/or anodic treatments for protection can be effective. It's also important to use higher grade alloys so pitting corrosion can be further diminished. For example, stainless steel is especially susceptible to this form of corrosion, so it may not be the best material to use in environments where there are high levels of sodium chloride (e.g. seawater, freshwater or brackish water).

Alloys of aluminum and stainless alloys are also prone to pitting corrosion, so these materials should be avoided in harsh, highly acidic environments.

Another potentially catastrophic form of corrosion is called stress corrosion cracking. As its name implies, stress corrosion cracking develops when weight-bearing forces are introduced or are a constant contributor. When combined with high temperatures, cracking can develop. The difference between it and pitting corrosion is the direction of the resulting fissures (perpendicular as opposed to vertical) and the environments in which it is most common. Generally speaking, stress corrosion cracking is most common in heavy machinery-related industries such as welding, grinding, pulverizing and cold forming. 

Rust may be a reality, but it's not one you must resign yourself to. For high quality environmental testing equipment services and unsurpassed capabilities in corrosion detection and prevention, choose Auto Technology. We have the experience and equipment that can suit your needs. Contact us today.