Why stainless steel rust
Stainless steel is the most durable of metals. Its mechanical properties enable its structures to remain extremely resistant to rust. Nevertheless, corrosion can’t be precluded. But there are ways to minimize the risk of corrosion.
Stainless steel’s fine layer of chromium oxide has natural techniques to self-repair when breached or broken. However, if the damage is too extensive corrosion can occur. There are various types of corrosion to be aware of.
The most common types are:
Pitting Corrosion on stainless steel
The passive layer on stainless steel can be attacked by certain chemical species. Chloride ion is the most common of these and is found in everyday materials such as salt and bleach. Harsh pitting corrosion is a localized damage that eats pits into stainless steel. In addition to chloride ion, can be caused by elevated temperatures for extended amounts of time or lack of oxygen to the surface.
You can avoid pitting corrosion by ensuring that stainless steel does not come into prolonged contact with harmful chemicals or by choosing a grade of steel which is more resistant to attack.
Crevice Corrosion on stainless steel
Stainless steel requires a supply of oxygen to make sure that the passive layer can form on the surface. In very tight crevices it is not always possible for the oxygen to gain access to the stainless steel surface thereby causing it to be vulnerable to attack. Crevice corrosion is avoided by sealing crevices with a flexible sealant or by using a more corrosion resistant grade.
General Corrosion on stainless steel
Normally stainless steel does not corrode uniformly like ordinary carbon and alloy steels. However, with some chemicals, mainly acids, the passive layer may be attacked uniformly depending on concentration and temperature and the metal loss is distributed over the entire surface of the steel. Hydrochloric and sulphuric acids at some concentrations are particularly aggressive towards stainless steel. General corrosion can be quite destructive and happen to the entire surface at once.
Galvanic corrosion on stainless steel
If two dissimilar metals are in contact with each other and with an electrolyte (e.g. water or another solution), it is possible for a galvanic cell to be set up and to accelerate corrosion of one of the metals. It can be avoided by separating the metals with a non-metallic insulator such as rubber.
Less common types of corrosion are:
Stress Corrosion Cracking (SCC) on stainless steel
Stress Corrosion Cracking is a relatively rare form of corrosion, which requires a specific combination of tensile stress, temperature and corrosive species, often the chloride ion, for it to occur. SCC can occur typically in hot water tanks or swimming pools. But if it does happen, it can be rapid, breaking down the mechanical properties of steel in days rather than months or years. Another form known as sulphide stress corrosion cracking (SSCC) is associated with hydrogen sulphide in oil and gas exploration and production
Intergranular Attack on stainless steel
Intergranular Attack is also a quite rare form of corrosion. If the carbon level in the steel is too high, chromium can combine with carbon to form chromium carbide. This occurs at temperatures between about 450-850 C. This process is also called sensitisation and typically occurs during welding. The chromium available to form the passive layer is effectively reduced and corrosion can occur. This corrosive harm happens between the grains and can be avoided by using a low-carbon stainless steel or uniform heating and rapid cooling of the steel.
Reba Scientific thanks Montanstahl of Switzerland for permission to publish this article.
The difference between pickling and passivation?
Though pickling do improve corrosion resistance but never to the same degree as passivation itself. The chart on the right is self-explanatory. Pickling is the removal of any high temperature scale and any adjacent low chromium layer of metal from the surface of stainless steel by chemical means. While, Passivation is the treatment of the surface of stainless steels, often with acid solutions (or pastes), to remove contaminants and promote the formation of the passive film on a freshly created surface (e.g. through grinding, machining or mechanical damage). After pickling, passivation should be done to obtain maximum corrosion resistance.
How to start your own passivation even if you don’t know anything?
The CitriSurf® citric acid cleaning and passivation process is based on cutting edge technology using safe and environmentally friendly citric acid to remove the free iron from the surface of stainless steel.
Stainless steel is comprised of a mixture of metals, primarily iron, chromium, nickel and in some cases molybdenum and manganese along with other materials in small quantities. It is this unique blend that allows the special property of corrosion resistance by forming an outer protective passive oxide layer. Stainless steel surfaces can be made highly resistant to corrosion if they are properly cleaned and passivated prior to use. Even with 300 series austenitic stainless steel, once a corrosion site has started, it will only get worse, being continuous and self-catalyzing. In corrosive environments the growth of the corrosion site will accelerate rapidly! Thus, proper cleaning and passivation of surfaces prior to use is essential to achieve maximum resistance to corrosion.
CitriSurf technology has proven to be a vast improvement over the traditional nitric acid formulations.
After stripping the iron from the surface, the citric acid forms a water soluble complex with the iron ions, tying them up so that they are no longer able to have a negative impact. The CitriSurf bath will not allow the iron to precipitate again like nitric acid is known to do. In addition, citric acid does not pose the health and environmental hazards that are present with nitric acid. By their very nature, CitriSurf products are non toxic, non-corrosive and biodegradable. Citric acid is the natural organic acid found in oranges and other citrus fruits. It is Generally Regarded As Safe (GRAS) and is commonly used in many foods and beverages. This means that CitriSurf is compatible with the environment and can usually be disposed into sanitary sewage systems with minimal waste treatment to meet environmental agency approvals.
Conversely, nitric acid is very corrosive by nature, represents significant personnel hazards, and threatens property as well. It is also an oxidizer which increases risk and expense. Heated, its risk is intensified. After use, expensive waste treatment is necessary prior to disposal, and can expose the user to long term liability under regulatory agencies.
CitriSurf solutions represent the state-of-the-art technology that is highly effective in stainless steel passivation. After many years of use, testing, and data collection, it is apparent that CitriSurf products produce superior results at lower overall cost, and are safer and more environmentally friendly. The old constraints of nitric acid date back many years, and are no more effective now than when initially developed at a time when the chemistry of passivation was little understood.
The use of nitric acid exists today only because it was long accepted as the “way to do it” through the years, including the use of Federal Specification QQ-P-35C as the industry standard. Now that QQ-P-35C has been replaced by consensus industry standards ASTM A967 and AMS 2700, it is certainly time to switch to the safer, higher performance CitriSurf system. With current technologies available for study of the surface chemistry of stainless steel, we can see that it makes sense to choose a material that eliminates the iron from the surface more completely and helps to produce a truly passive surface by creating the best possible chromium layer.
CitriSurf solutions provide the maximum protection by providing:
- Improved removal of free iron from the surface
- Faster removal of free iron from the surface
- Low hazard chemistry
- Environmentally safe chemistry
- Lower cost
- Passivated surfaces that pass all salt spray, immersion and high humidity tests
Industries That Use Citrisurf
CitriSurf® can be and is used in any industry where stainless steel is found, including but not limited to:
- Parts Manufacturing
- Metalworking and Fabrication
- Process Cleaning
- Facilities Maintenance
Food and Beverage
- Food Process Equipment
- Brewery, Winery, Distillery, and
- Other Beverage Process Equipment
- Dental and Orthodontic Instruments
- Surgical Instruments
- Medical Equipment
- Orthopedic Implants
- Process Equipment
- Equipment Maintenance
- Rig Maintenance
- Semiconductor Equipment
- UHP Devices
- Electrical Enclosures
- Damaged Stainless Kitchen Appliances
- Brass and Bronze Tarnish Removal