Stainless steel is renowned for its corrosion resistance. Although various stainless grades offer a wealth of useful attributes, it is generally deployed to resist chemical attack in challenging environments. Oil and gas manufacturers, for instance, use stainless steel for a raft of critical components, including: pipelines, heat exchangers, oil platforms, storage vessels, and more. However, despite its exceptional performance, stainless steel is far from impervious to all forms of corrosive deterioration.

A Brief Overview of Stainless Steel

Stainless steel has a carbon content of around 1.2%, nickel content of about 35%, molybdenum content of 5%, and chromium content of over 10.5%. The chromium provides increased corrosion resistance through an additional layer of defense once it reacts to oxygen.

Pitting Corrosion

When stainless steel experiences pitting corrosion, its surface has suffered from holes because of the corrosion. These pits are extremely small and hard to see. This is what makes this type of corrosion so hazardous. Very little of it can be seen upon the surface of the stainless steel. The holes can travel quite far into the material, meaning it becomes less strong and experiences damage.

What Causes Pitting Corrosion?

Pitting corrosion occurs when the layer created by chromium on the stainless steel begins to weaken. This means that it is more vulnerable to chemicals like sodium chloride, also known as salt, or halide ions. Hydrochloric acid, bleach, and deicing salts can also be responsible for pitting corrosion.

Other ways that pitting corrosion can occur include the:

  • Creation of scratches, chips, and cracks on the surface of the material, which can harm the layer provided by the chromium
  • Surface experiencing non-uniform stress
  • Layer being damaged because of conditions with a high or low pH
  • Appearance of inclusions or impurities.

Signs of Pitting Corrosion

Indications of pitting corrosion include the emergence of pits within the surface or a change in the look of the surface, such as its smoothness. The appearance of flash rust can also show that pitting corrosion has occurred. This is because it demonstrates that the particles of iron have remained upon the material.

If there is suspicion of pitting corrosion, detection methods are available to confirm its existence. This includes penetrant dye testing, scanning electrochemical microscopy, and digital holographic microscopy.

The Effects of Pitting Corrosion

Unfortunately, when pitting corrosion has begun, it can worsen extremely quickly, especially as it is hard to initially identify. Pitting corrosion can cause the following issues:

  • The creation of localized damage
  • The possibility of contamination if an area cannot be cleaned or experiences corrosion
  • The formation of concentrations of stress, which can potentially create cracks
  • The deterioration of the thickness of the material, which can lower the strength of its structure.

Preventing Pitting Corrosion

The type of stainless steel you select will determine how well it can resist corrosion. Utilizing a grade of stainless steel with increased amounts of molybdenum, like 316 stainless steel, is better at withstanding corrosion. Another way of lowering the risk of pitting corrosion is through incorporating additional protection, like a:

  • Coating
  • Paint
  • Surface treatment.

It is also important to manage the environment where the stainless steel is applied. That way you can ensure that its defensive layer will not be damaged.

Crevice Corrosion

Pitting corrosion can be seen upon the surface of stainless steel. However, crevice corrosion occurs when the spaces found between various surfaces begin to hold water or a different mixture. When these gaps lose oxygen, its conditions become acidic. This is when corrosion can occur and reduce the protection provided by the defensive layer of the stainless steel.

What Causes Crevice Corrosion?

Crevice corrosion appears when there are low amounts of oxygen within small gaps or crevices. As a result, the stainless steel’s chromium layer stops reforming. This type of corrosion can also occur due to the presence of moisture.

Signs of Crevice Corrosion

It can be difficult to see the damage that crevice corrosion inflicts upon stainless steel. However it can quickly spread within this material. Therefore, it is important to keep an eye on stains of rust. As internal devices, such as tubing clamps, can be affected by crevice corrosion, they may need to be taken out of the overall system.

The Effects of Crevice Corrosion

If left untreated, stainless steel can quickly deteriorate because of corrosion. The consequences of this corrosion are normally worse than those from pitting corrosion. This is due to the gaps being more extensive and thinner. Moreover, the thickness of stainless steel components can be affected by crevice corrosion and may break under weight. Cracks may also form as crevice corrosion can increase specific concentrations of stress.

Preventing Crevice Corrosion

The issues from crevice corrosion stem from the stainless steel having gaps within it. Creating a design for a device that incorporates fewer gaps between surfaces, such as those found next to corners and bolted connections, is one way to deal with crevice corrosion. Should this not be possible, the following techniques can help:

  • Applying cathodic protection 
  • Incorporating coatings and inhibitors
  • Using seal crevices, such as polysulfide sealants.

Locating a Robust, Corrosion-Resistant Steel Can Help with Pitting and Crevice Corrosion

Pitting and crevice corrosion can have an effect on stainless steel if precautions are not taken to strengthen it. When searching for stainless steels, it is important to consider its application and levels of resistance. For stainless steel used in conditions that could cause pitting and crevice corrosion, you may want to apply 316 stainless steel. However, for more challenging environments, a stainless steel alloy with increased levels of resistance, like super duplex, is better. Here at Masteel, we have all these grades of stainless steel available. Browse our website to learn more about 316 stainless steel, UNS S32750 SuperDuplex stainless steel, or UNS S32760 SuperDuplex stainless steel.