Stainless steel is a corrosion-resistant metal especially designed for industrial use. It is non-magnetic, has a relatively high melting point, and maxes out on the AISI 1040 grade. There are two main causes that lead to its corrosion: chemical reactions from salt or potential oxidation from atmospheric oxygen that produces chlorides and when it gets exposed to water and certain other chemicals. 

The article covers the early signs that can be taken note of to prevent rusting, how to tighten these signs up before they become major problems, and how to properly care for your stainless steel.

TYPES OF CORROSION
General corrosion
(or uniform corrosion)
  • Uniform loss of metal over entire surface
  • Stainless steel with pH value of less than 1 are more prone to general corrosion
 Galvanic corrosion
(or bimetallic corrosion)
 
  • Electrochemical process where one metal corrodes preferentially compared to another in the presence of electrolyte
 Intergranular corrosion  
  • Corrosion where boundaries of crystallites are more likely to corrode than inside surfaces
  • Occurs after heating austenitic stainless steel at approximately 842–1562°F
 Pitting corrosion  
  • Localized corrosion resulting in cavities or holes
  • Occurs when stainless steel is exposed to environments containing chlorides
 Crevice corrosion  
  • Localized corrosion at the crevice between two joining surfaces
  • Formed between two metals or between metal and non-metal
 Stress corrosion cracking  
  • Growth of crack formation in corrosive setting
  • Tensile stresses in combination with corrosive environmental conditions lead to cracking

It is useful to understand how stainless steel alloys vary from other steels in order to understand why stainless steel is rust-resistant and how this resistance breaks down.

Stainless steel has at least 10.5 percent chromium. This chromium immediately interacts with the surrounding oxygen, forming a thin oxide coating on the steel’s surface. Unlike iron oxide, which often takes the form of flaky and corrosive rust, chromium oxide adheres to the steel. As a result, it serves as a protective barrier. The chromium oxide acts as a passive layer, keeping the iron in the alloy isolated from the air and water in the environment. This coating is responsible for stainless’s rust resistance.

Stainless steel requires little to negligible maintenance and is resistant to oxidation and staining, making it a great material for a wide range of applications.

Stainless steels are classified into four categories: austenitic, ferritic, martensitic, and duplex. Austenitic stainless steel dominates the industry, accounting for more than 70% of total stainless steel output. Its features include a maximum of 0.15 percent carbon and a minimum of 16 percent chromium, resulting in very good rust resistance. Ferritic stainless steel has lower corrosion resistance than austenitic grades but outperforms martensitic stainless steel. Duplex stainless steels have excellent resistance to localized corrosion, including pitting, crevice corrosion, and stress corrosion cracking.

Four Factors That Causes Stainless Steel To Corrode

  1. Pitting Corrosion in Stainless Steel Can Be Caused by Strong Chlorides:-

When exposed to chloride-rich conditions, several kinds of stainless steel alloys will experience severe pitting corrosion (such as salt). When used in marine applications, for example, grade 304 stainless steel may begin to pit as a consequence of contact with salt water (which is high in salt) or salt-enriched sea breezes.

To minimize pitting corrosion, utilize a grade of stainless steel that is resistant to chlorides, such as grade 316 stainless steel. To avoid direct contact with chlorides in the environment, a specific coating might be put on the steel.

  1. Welding Dissimilar Stainless Steel Alloys Causes Bimetallic/Galvanic Corrosion:-

One common error that some manufacturers make when constructing a bespoke steel wire or sheet metal form is welding two different metals together, whether by accident or intentionally.

The easiest way to avoid bimetallic corrosion is to avoid permanently combining two different metals in the first place. A close second is to cover the metals with a coating to seal them off and impede electron transfer from the cathode to the anode.

It should also be noted that utilizing a weld filler that is too different from the metals being connected might result in galvanic corrosion at the weld site.

  1. Plain Iron or Steel transplantation onto Stainless Steel:-

Particulate residue from a plain steel or iron workpiece may be deposited onto the surface of a stainless steel component or basket in certain situations. These simple iron or steel particles may damage a stainless steel workpiece’s protective oxide coating, reducing its corrosion resistance and causing it to rust.

The difference between this and the previously mentioned bimetallic corrosion issue is that in this scenario, the contact between the different metals is totally incidental and usually occurs without the manufacturer’s awareness.

To avoid the transplantation of plain steel or iron (or any other metal) to stainless steel workpieces, it is essential to fully clean and prepare equipment before switching to new material. Some tools, such as steel brushes, should never be exchanged across metal kinds.

  1. Using Extreme Temperatures on Stainless Steel:-

Stainless steel alloys often have a very high melting point (far beyond 1,200 F). While the metal does not melt at high temperatures, it may undergo other modifications that influence its corrosion resistance.

Scaling, for instance, is a typical issue with stainless steel alloys when subjected to high temperatures (such as those employed in many heat treatment/annealing procedures). When scales develop on heated metal, the flaky outer core may induce bimetallic corrosion since the scales are not the same as the underlying metal.

Extreme heat may also cause uncovered stainless steel alloys to shed their protective oxide coating for a period of time, increasing the risk of corrosion until the oxide layer can re-form.

To avoid corrosion from scaling or other difficulties caused by temperature extremes, verify the recommended operating temperatures for each stainless steel to determine whether the temperatures utilized in your production processes exceed those limitations. 

Best practices to help prevent rusting:-

Stainless steel corrosion prevention measures should be used throughout the stainless steel lifespan. Best practices in the design and manufacturing stages, as well as frequent maintenance, will extend the metal’s performance and aesthetics.

1. Designing

Long-term benefits come from a strategic approach to stainless steel design. Water infiltration and deformations may be reduced with careful management at the design stage of stainless steel applications. Water drainage holes should be utilized wherever feasible, and cavities and cracks should be kept to a minimum. Airflow is vital, and the structure should allow for unobstructed circulation of air throughout the operation.

2. Production Stage

It is critical to avoid touching stainless steel with iron or ordinary steel during the fabrication stage. This necessitates constant observation of the external setting, which includes work tables, tools, storage units, steel turning rollers, and chains. Any carbon steel dust particles that settle onto the stainless steel surface during manufacture might contaminate it, increasing the possibility of rust development. Cleaning and grinding equipment used with carbon or low alloy steel must also be completely separated from stainless steel.

3. Management

Regular maintenance is essential for preventing stainless steel rust and slowing the advancement of existing rust. It is essential to remove any rust that has developed, either mechanically or chemically. The filth that forms may then be removed with warm water and soap. A rust-resistant coating should be applied after washing. 

Other household or outdoor stainless steel surfaces can be prevented from rusting by following the steps:- 

Prevent metal from rusting in all types of moist environments, by using proper heat protection such as stainless steel bimetallic strips; stainless steel surfaces are prone to rust if exposed to corrosive elements. To prevent this from happening, it is best to wipe down these items with a solution of 3% hydrogen peroxide, this helps remove any moisture that could lead to rusting. Additionally, it is recommended to coat a thin layer of oil to extend the life of your stainless steel products. 

For larger products, a blow torch is required to clean off any corrosion. Scrubbing surfaces with a Brillo pad or steel wool can also be proven to be effective. 

Stainless steel products at Prashaant Steel:-

Stainless steel is known for its properties that make it resistant to rust. At, Prashaant Steel, we take utmost care and implement scrutinized procedures to ensure quality production of stainless steel. To find out more about our products, reach out to us. 

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