Stainless steel is an iron alloy containing elements like carbon, silicon, manganese, phosphorus, sulfur, nickel, chromium, molybdenum, copper, nitrogen, and others. These elements altogether form stainless steel. In addition to this, these elements also make the steel lustrous, resistant to corrosion, and of low maintenance. Due to this, stainless steel is one of the most preferred choices for many applications.
So, Does Stainless Steel Rust? No, stainless steel does rust. In the scarcity of chromium on the surface of the stainless steel, a layer of chromium oxide does not form making the outer area vulnerable to rusting and can turn the color to reddish-brown.
Though all other elements prevent rusting of stainless steel in one way or another, its nickel and molybdenum which comes after the chromium. You might have realized by now that averting rust formation is all about whether chromium is present near the surface or not. Also, at what percentage elements are present in the iron alloy.
The ideal percentage of the essential elements that prevent the formation of rust:
- Chromium: must be 10.5 to 11 or above
- Nickel: must be 8 or above
- Molybdenum: approximately around 5
It is to be noted that the elements of stainless steel provide other abilities such as hardening, discoloring, scratch-free, malleability, ductility, and many others.
You might be surprised to know the fact that stainless steel is not completely stainless. It is a mere gimmick to increase sales.
Why stainless steel is not corrosive?
Stainless steel does not undergo corrosion when exposed to air and moisture. It is because chromium undergoes passivation through which it forms a thin layer of chromium oxide.
This layer forms when chromium reacts with the oxygen present in the air and the water. Additionally, chromium oxide prevents oxygen from associating further with the steel preventing it from spreading throughout the metal.
It might be interesting to know that a thin layer of chromium oxide is self-repairing in nature. It means, in case of its disruption, chromium again reacts with fresh oxygen molecules to fil the gap. This is the reason why stainless steel does not corrode easily.
Once the chromium oxide layer on the surface of the stainless steel is disrupted, a normal chemical reaction of rusting on steel takes place.
Below is the chemical reaction of the rusting process.
4Fe + 3O2 + 6H2O ——–moisture and air——–> 4Fe (OH)3 (Iron Hydroxide)
Iron Hydroxide dehydrates to become Fe2O3.H2O which is also called rust.
By this time, you might be thinking why there are no single ideal set percentages of all elements for the stainless steel?
Having a predetermined number for chromium, nickel, and molybdenum can help with preventing rust formation and will act as a single guideline throughout the globe? No, this is not how it works.
Major factors of corrosion
The aforementioned image shows all major reasons because of which stainless steel undergoes corrosion.
Real-life examples of rusting of stainless steel
Some of the real-life examples of rusting of stainless steel can be:
- Rust on a food can
- Rust on water jug and bottles
- Corrosion of computer circuit board
- Rust on pipe fittings in washroom and kitchen
- Rust on the railing of a nearby park
Think about a situation where you might have left a paper clip in the damp sink of your kitchen. If left overnight, the next day you will find a slight discoloration on the sink, in the shape similar to that of the paper clip. Here, the paper clip prohibited the formation of chromium oxide and rusting initiated instantly.
Now, think about other situations for the aforementioned everyday examples.
Graphical representation of the effect of chromium on the corrosion rate of the stainless steel
It can be seen from the graph that a lesser chromium percentage has higher values of the corrosion rate. Also, after a certain value of chromium percentage, the corrosion rate became constant.
Graphical representation of the effect of temperature on the corrosion rate of the stainless steel
From both the graphs, it can be analyzed that the corrosion rate is directly proportional to the temperature. The higher the temperature is, the higher will be will corrosion rate.
Graphical representation of the effect of pressure on the corrosion rate of stainless steel
From the graph, it can be seen that higher temperature and lower pressure or vice versa conditions can be handled by the stainless steel without undergoing corrosion. Any alteration to this with an increase in time will lead to the formation of rust.
Graphical representation of the effect of different geographical locations on the corrosion rate of stainless steel
The four selected geographical locations in the graph have different geographical conditions. Quintero has a temperate climate and Laja has a dry and rural environment. Whereas, Arica and Antarctic have cold and marine environment.
From the graph, it can be seen that Quintero having a temperate environment has a maximum corrosion rate.
Both Arica and Antarctic have similar corrosion rates and have values quite closer to one another. Lastly, Laja having a dry and rural environment has the least amount of corrosion rate values.
Types of stainless steel
Stainless steel is useful for different purposes for which elements are added in different compositions as per the requirement. Some need stainless steel to be weldable while others need solid and hard metal. The elements of stainless steel not only prevent rusting but contributes to other specialties as well.
Due to this reason, there are five main types of stainless steel because of its crystalline structure.
- Austenitic stainless steel
- Ferritic stainless steel
- Martensitic stainless steel
- Duplex stainless steel
- Precipitation hardening stainless steel
Let’s discuss all one by one!
Austenitic stainless steel is the most common type and has high percentages of Nickel, Nitrogen, and Manganese.
They are highly weldable, ductile, and does not harden by any heat treatment. Some of the examples can be household utensils, spring, or knife.
Ferritic stainless steel has a high percentage of chromium and less carbon (lesser than 0.10 percent). They are not weldable and lack toughness. They do not harden by any heat treatment.
As they have higher concentrations of chromium and molybdenum, they can be used under harsh conditions like seawater. Also, they do not corrode easily. Making nut and bold out of this metal is the best option.
Martensitic stainless steel has a chromium percentage similar to ferritic stainless steel but has a carbon percentage of up to 1 percent.
Due to the high percentage of carbon, they are used where high strength and average corrosion resistance is required.
They have low weldability because of which are used in wear-resistant equipment, turbines, aerospace applications, hydroelectric power plants, and others.
Duplex stainless steel has high percentages of chromium and molybdenum but lower percentages of nickel. The duplex steel has 50 percent properties of ferritic steel and 50 percent properties of austenitic steel.
Due to this, it has approximately twice yield strength than the austenitic steel and has better resistance to corrosion cracking.
The duplex stainless steel is the most suitable choice for paper and pulp industry as well as the oil and gas industry.
Precipitation hardening stainless steel has copper, aluminum, and niobium in it. All these elements make precipitation hardening stainless steel of very high strength.
This steel precipitates at very high temperatures which form small particles in the matrix of the steel. These minute particles impart strength and a good tolerance power to heat.
Due to this, this steel can be used to form intricate shapes as they undergo minimal to zero distortion before reaching the final treatment. It is a superalloy that is used in jet engines, turbo parts, gas turbines, and others.
Precipitation hardening steel can bear high temperature with undergoing menial distortion!
Massive global production
The stainless steel has an ever-growing production which you can see from the graph. The production reached 52.2 million metric tons in 2019, which was mere 24.5 million metric tons back in 2005.
One main reason of such surge in production is, both the North American Federal Trade Agreement (NAFTA) states and the Commonwealth of Independent States (CIS) have undergone upheaval both in production and consumption of the stainless steel.
Out of these numbers, the USA has a small footprint when it comes to the production of stainless steel.
The production of stainless steel by the USA has reached 2.8 million metric tons by 2018, which was 2.2 in 2010. It can be seen that within 8 years of span, not much increment in production has occurred.
Other statistics released by the International stainless steel forum shows foreign trade export of stainless steel for the year 2019, inclusive of feedstock material.
Who is the largest producer of stainless steel in the USA?
It is Nucor Corporation as an issue by the American Metal Market.
Does America import stainless steel?
Yes, the USA was the world’s largest importer of stainless steel in 2019. The USA imported approximately 26.3 million metric tons of steel in 2019, which was 30.8 million metric tons back in 2018.
Steps to avoid rusting of stainless steel
- Keep your environment clean and dry (rust form in air and moisture)
- Try applying a protective coat of potassium nitrate, water, and sodium hydroxide
- Try using liquid zinc as a coat (this process is called as galvanization)
- Try using baking soda on household appliances
- Regularly maintain your equipment and scrape the rust off as soon as possible
- Oxalic acid can also be used to clean the outer surface
Some other related questions
Is stainless steel recyclable?
Yes, stainless steel is 100 percent recyclable. A single production lot is made of approximately 60 percent of recycled material and 40 percent of end of life products.
Does stainless steel have a carbon/ ecological footprint?
Stainless steel has approximately 2.90 kg of co2/ kg of stainless steel.
Does stainless have any degrading impact on human health?
Welding and cooking are two major sectors that are discussed rigorously. It has been criticized that welding stainless steel produces carcinogenic fumes that can lead to cancer. These fumes are categorized as Group 1 carcinogens by Cancer Council Australia.
For cooking, it has been proven scientifically that stainless steel makes food highly acidic due to the elements like chromium and nickel.
Eating such food for a longer time can introduce carcinogens and oncogenes in the body and reduces the overall immunity of the body.
Stainless steel being an iron alloy does rust and corrode. It is all about the percentages of chromium, nickel, molybdenum at which they are mixed in the alloy. The rusting starts when the surface of the metal comes in contact with the air and moisture.
So, if a large percentage of chromium is present near the surface in mixture with nickel and molybdenum, rusting of the metal can be prevented.
It is said that oxalic acid removes rust on the stainless steel but don’t you think prevention is always better than cure. Next time, buy stainless steel as per your need.
So guys, if you have any doubt regarding the rusting of stainless steel. You can ask them in the comment section. We will reach out to you quickly.