Reducing the magnetic, thermal, and electrical conductivity of metals, making jewelry unattractive and unusable, and posing as a safety hazard in infrastructures and transportation—these are just some examples of the negative effects. Metal corrosion comes in different forms and rusting is one of them. Humans have found out different ways on how to make metals resistant to it, which we all take advantage of to produce safe products that will last for years (or decades).
However, resistance is not equivalent to immunity. Metals and alloys can still corrode after several years, although some can last up to centuries. Therefore, it’s important to know which types rust and corrode quickly and which ones can remain in good condition for years. This way, you’ll know what type of material to look for when buying certain items, so you can rest assured that it won’t be a problem for you any time soon.
What causes it?
When the iron in the metal comes in contact with air and water (or moisture), an electrochemical reaction occurs, resulting in the formation of an oxide. Rust is the product of such reaction, an iron oxide in its hydrated form. Depending on the type of iron oxide formed, the color may vary: it can be orange, red, yellow, brown, or a mix of these colors. We’re most familiar, however, with red rust, which is iron (III) oxide trihydrate or hydrated ferric oxide with a chemical formula of Fe2O3•H2O.
In a nutshell, this is what happens:
iron + oxygen (air) + water → iron (III) oxide trihydrate
If one element is lacking in the equation, the process won’t occur. Therefore, it only happens to materials that contain iron, and only if water or moisture is present. Water facilitates the transfer of electrons between the iron and the environment, which is basically the oxidation of iron. Let me discuss it briefly below:
- When exposed to a strong oxidizing agent such as oxygen, the iron readily gives up its electrons (e–), and the iron goes into the aqueous solution (water) as a cation (Fe2+).
Fe → Fe2+ + 2e–
Balanced equation: 2Fe → 2Fe2+ + 4e–
- The oxygen and water react with the surface of the metal, and the oxygen gets dissolved in the water, forming hydroxide ions (OH –).
O2 + 2H2O + 4e– → 4OH–
- And then, the iron ion (Fe2+) and the hydroxide ion (OH –) reacts to form iron hydroxide [Fe(OH)2].
2Fe2+ + 4OH– → 2Fe(OH)2
- Lastly, the iron hydroxide [Fe(OH)2] reacts with the oxygen to form red rust (Fe2O3•H2O).
This whole chemical reaction is summarized in the image below. The red brick represents the rust formed.
Image source: Spennemann, D.H.R. from Research Gate
The process is sped up when the metal is exposed to better electrolytes, such as saltwater and acidic solutions/environments. This is because there are more available oxidizing agents to attack the iron, corroding the metal at a faster rate.
Other metals also undergo this type of corrosion, but the result cannot be classified as rust, simply because those are not iron oxides. For example, silver reacts with sulfur and the oxide called silver sulfide is formed, which is commonly known as silver tarnish.
Steel is an alloy of iron and carbon and may also contain minute amounts of silicon, phosphorus, sulphur, and oxygen. With around 98-99% iron content, steel readily rusts upon exposure to water and air. This posed a problem in products made of steel since rusting decreases the products’ lifespans. So, to increase the resistance of corrosion and other damage, elements that protect it from oxidation were used to either coat the alloy or incorporated during steel manufacture.
Stainless steel is an example of an alloy resistant to rusting and corrosion; it contains a high percentage of chromium, usually around 10-30%. This element gives the alloy a remarkable increase in heat and corrosion resistance. How does chromium impart rust resistance?
Chromium is an element that is more reactive than iron. Meaning, before it can oxidize to form rust, chromium reacts first with oxygen to form an oxide layer on the surface. This acts as a protective layer on the surface to prevent oxygen from making contact with the underlying layer, thereby resisting oxidation and corrosion.
Alongside chromium, other alloying elements such as nickel, molybdenum, titanium, aluminum, etc., are also added to increase corrosion and resistance to specific environmental conditions or to impart other characteristics (e.g., ductility, tensile strength, electrical conductivity, etc.).
Does it tarnish?
As mentioned, materials with iron in them can rust, and since stainless steel has this in it, the process can occur. However, it is important to note that due to the previously mentioned components of this alloy, stainless steel does not rust nor tarnish readily, which is why it’s called “stainless.” The protective layer prevents oxidation, thus prevents the formation of tarnish, too. This makes this alloy one of the most widely used materials for engineering and practical products.
It will take continuous or frequent exposure to unfavorable or extreme conditions such as contact with damaging chemicals, saline, heat, and moisture before it will corrode and get damaged. Mechanical damage (i.e., scratches) that compromises the chromium oxide layer can also impart faster corrosion and oxidation.
If you purchased something that’s supposedly stainless steel, but it tarnishes within the month or so, then you must have been tricked into thinking that the item is made up of this superior material. Most probably, the metal is just coated thinly with something that will make it look like it. I suggest reading the item’s reviews and for you to buy from highly reputed stores.
How to remove it from stainless steel appliances
To prevent rust from forming on stainless steel appliances, you must do regular cleaning and maintenance. First and foremost, you must wipe off any residual moisture (if possible) from the item after use, or you can air out the bathroom/kitchen/room to let the moisture dry on its own. Also, make sure to avoid exposing the appliances to rain and direct sunlight.
It is important to remove any developing rust immediately to avoid further degradation. Take note that you must avoid using chlorides (bleach), abrasive, and all-purpose cleaners on stainless steel since these will damage the alloy. Here are some examples of routine cleaning processes that you can do every 3 months to lengthen the lifespan of your appliances (from Reliance Foundry):
- For appliances with oil and grease marks:
- saturate a soft cloth with solvents like isopropyl alcohol or acetone
- rub the saturated cloth on the affected areas until all oil/grease marks are gone
- wash the item with mild soap/detergent and rinse properly with water
- wipe dry with a soft cloth
- For appliances with discoloration
- make a baking soda and warm water mixture (1:1 ratio) or washing soda and warm water mixture (also 1:1 ratio), and apply it to the affected areas
- let the mixture stay on for a few minutes (15-30 mins in my opinion, but you can extend it if the discoloration is stubborn)
- rinse the item with water and wash with mild soap
- rinse with water and dry properly
- Localized stains
- soak a soft cloth with an oxalic acid solution or other commercially available removers
- apply the solution onto the affected areas
- leave the oxalic acid on for a few minutes until it dissolves the particles; for removers, follow the instructions that it comes with carefully
- rinse the item properly, making sure no solution is left, with water
- dry the item with a soft cloth
If you don’t want to go through such lengths (and extra cost) to avoid it, make sure that any vulnerable item is kept away from moist or wet environments. Clean items made up of metal regularly to avoid tarnishing. If you notice your home frequently gathering dust, I suggest you look into UV light sanitizers for your HVAC and furnace systems to avoid multiple exposures of your tool to water due to frequent cleaning.
Lastly, store all items that are susceptible in areas with proper temperature and humidity. If you noticed that your central home AC is not blowing cold air, you should resolve it quickly. Hot and humid areas do not bode well with avoiding rust.
Whereas stainless steel has a higher amount of chromium, this type, on the other hand, has a higher concentration of carbon (up to 2.5%) than the regular type (0.05-0.3%). The increase in carbon content imparts toughness and strength, forming a strong crystal lattice of carbon molecules. This cross-linking of carbon molecules gives it high stability even at high temperatures, which is ideal for high-temperature applications such as waste oil heaters.
Just like other types, carbon steel contains iron; thus, it can rust when exposed to water and air. This alloy may also contain other elements like molybdenum, chromium, nickel, etc. It, however, does not form a protective oxide layer upon exposure to air.
Depending on the carbon content (wt.%), its properties may differ:
- Low-carbon (<0.25%) – high ductility and toughness; low hardness and cost
- Medium-carbon (0.25-0.60%) – medium strength, ductility, and toughness; low hardenability
- tough; resistant to wear
- harder to weld, cut, and bend
- used for railway tracks, machinery, gears, etc.
- High-carbon (0.6-1.25%) – high strength and hardness; low ductility
- high wear resistance; extremely hard and brittle after heat treatment
- used for cutting machinery, high-strength wires (music), springs, etc.
High-carbon steel is indeed a good material for cutting machinery, but it will not be able to handle tougher metals like tungsten or titanium—you better use plasma cutters instead!
Carbon fiber is made up of organic polymers, characterized by long, thin, crystalline filaments of carbon. The arrangement of carbon molecules makes it five times stronger than steel, but unlike the latter, it is easy to manipulate to suit different applications. It also remains lightweight (5 times lighter than steel and 2 times lighter than aluminum) and has high tensile strength, which makes it ideal for many manufacturing processes.
What’s great is that carbon fibers are not as affected by heat compared to other metals (a.k.a. low thermal expansion) and are resistant to rust and corrosion. It has an epoxy resin thatis chemically inert and will not react with oxygen, thereby preventing oxidation. Here are some applications of carbon fibers:
- Watersports accessories/products
- Drone parts
This type is used in biomedical applications, a variety that prioritizes corrosion and oxidation resistance over strength. This alloy does contain iron, but it also has other alloying elements like chromium (usually at least 13%), nickel, molybdenum, and carbon to resist corrosion. Examples of surgical stainless steels are austenitic 316 stainless and martensitic 440 and 420 stainless.
Since it comes in contact with bodily fluids, it is important that it is resistant to oxidation. Examples of items that must be made up of this material are the following: forceps, needle holders, piercing needles, dental implants, orthopedic implants, etc.
Aluminum, the most abundant metallic element in the Earth’s crust, is a lightweight, low density, non-toxic metal that is soft and malleable and has excellent thermal and electrical conductivity. This silvery-white material is one of the most important and widely used nonferrous metals. It is used in a vast array of applications:
- as a coating or component of food containers and kitchen utensils;
- as a reflective coating for light and heat;
- manufacture of electrical transmission lines; and
- manufacture of parts for various forms of transport (airplane, bikes, cars, etc.)
Alone, aluminum is too soft to be used in the manufacture of airplane parts and other forms of transport, but when alloyed with other elements like copper, manganese, magnesium, and silicone, it becomes strong but stays lightweight.
Does aluminum rust?
Since aluminum is a nonferrous metal, it does not. Even aluminum alloys do not usually contain iron, so those won’t either. However, just like other metals, aluminum does oxidize and form an oxide upon reacting with oxygen. It is actually even more reactive than iron, so it oxidizes faster. If this is the case, then why is it used in the manufacture of important items such as airplane parts?
This is because, upon oxidation, the aluminum oxide that forms on the surface protects the metal from further oxidation and corrosion. This hard layer acts as a resistant shield that prolongs the lifespan. Further study even noted that exposure to water actually strengthens the aluminum oxide even more. This is the opposite from the case of other metals wherein their corrosion processes speed up when exposed to water.
It turns out that when the aluminum oxide layer comes in contact with water molecules, its structure changes and becomes chemically inert. Thus, it will not react anymore with other water molecules or atmospheric oxygen, making it corrosion-resistant. This is why aluminum can be used in the manufacture of fuel transfer tanks and waste oil heaters; whether you store oil or water in it, you won’t have to worry about any unwanted reactions.
However, despite its high inherent resistance to oxidation and corrosion, aluminum is not immune to all damages. It also needs regular maintenance to make sure that the oxide layer is not compromised. Usually, the oxide layer will repair itself once exposed to oxygen, but when it is exposed to chlorides and sulfides instead, it will be jeopardized. Saltwater and polluted air both pose a threat to aluminum oxide, containing chlorides (sodium chloride is salt) and sulfides, respectively. Without a protective oxide layer, aluminum will undergo various corrosions such as galvanic corrosion, pitting, etc.
If aluminum is maintained regularly and kept away from unfavorable conditions, it will stay strong and last for years. Make sure to also read our article on welding aluminum, as well as the various joints to choose from. If you do so, make sure you also have the right welding glasses to keep your eyes safe!
I have touched the process of galvanizing metals briefly in this article (please do hyperlink the previous article here) when I talked about zinc. Galvanized steel is basically carbon steel coated in zinc, wherein zinc acts as the sacrificial metal to protect the layer underneath. So, despite containing iron, it will not rust because the coating acts as a protection against corrosion and damage. Galvanizing can be applied in various fields due to different available galvanization methods; it is done to make corrosion-resistant material for roofing, automotive parts, etc.
How long does it take for galvanized steel to experience it?
Zinc is more reactive than iron, so upon exposure to oxygen, it will react first to form a layer of zinc oxide on the surface. This layer protects the iron from oxidizing, thereby preventing the damaging process. However, when the coating or oxide layer is compromised, moisture and air can come in contact with the layer underneath. This will lead to rusting.
If we are talking about the natural process of deterioration of the zinc coating, it will take years before the layer underneath rusts because the zinc oxide can repair itself when the zinc coating gets exposed to air. In average environments, properly galvanized steel can last up to 20-50 years, depending on the thickness of the coating.
However, if extreme/unfavorable environmental conditions and mechanical damage are taken into consideration, it may corrode quickly (even more so if the coating is thin). Examples of such conditions are saltwater, high temperature, and acidic environments.
By itself, zinc is pretty weak and brittle, with low to moderate tensile strength and conductivity. However, when alloyed with other elements, it results in the formation of alloys with high impact strength and ductility and has corrosion and rust resistance. As I’ve mentioned in the previous section, zinc acts as a sacrificial metal to form a protective layer of zinc oxide; therefore, even if alloyed with iron, it will take years before zinc alloys rust.
Copper is extremely ductile and malleable and has a high thermal and electrical conductivity. This is why it is widely used to create electrical wires, coins, motors, heat exchangers, etc. Since copper does not have any iron in it, it does not rust. It does, however, oxidize and undergo series of chemical reactions upon exposure to air, forming a green layer called patina.
But you don’t have to worry about this patina; it actually protects the copper from corrosion! This is why copper is also used in manufacturing roofing materials because the layer of patina protects the underlying material from moisture and air.
However, this patina cannot protect the copper from other forms of corrosion, such as pitting, galvanic corrosion, and other processes. Exposure to acids, heavy-metal salts, sulfur, and ammonia can corrode copper.
Tungsten is a highly durable metal, with the highest tensile strength (142,000 psi) and melting point (3687 K) among natural metals. Because of the durability of tungsten, it is used to alloy with other metals for added strength and corrosion resistance. However, pure tungsten does oxidize readily upon exposure to air, developing colorful tarnish or patina. This poses a problem especially in making jewelry.
This is why tungsten carbide, an alloy of tungsten and carbon with a nickel binder, is the one most widely used for high-quality jewelry. It does not readily oxidize/tarnish, except at very high temperatures (around 1100 K above), so you won’t have to worry about stains around your finger. Furthermore, this alloy does not contain any iron, so it will not rust. The durability of tungsten is also retained in tungsten carbide, imparting scratch resistance, and the nickel binder allows the alloy to hold a shape, so the jewelry will last longer. Due to its high durability, tungsten carbide is also used in making aircraft, tanks, bullets, metallic cement, etc.
How to remove it from metal in general
If rusting is only minimal and limited at the surface, mechanical and chemical means can be used to remove rust from metal. The most common mechanical option is to rub it off with fine steel wool or wire brush, and then rinsing the metal with warm soap and water. Another method is to scrape off stains with the use of electric wide belt sanders, which is a much efficient method to get rid of rust from large metal surfaces.
However, this method is best suited for scratch-resistant metals. Those with coatings are better off to be treated with various solutions or homemade concoctions instead.
Easy-to-make homemade solutions include:
- baking soda and warm water
- washing soda and warm water
- baking soda and lemon juice/vinegar
Just mix your combo of choice in a 1:1 or 2:1 ratio to make a paste, and then apply it to the affected areas. Leave it on for a few minutes to let the damaged part dissolve, rinse the item with water, wash it with mild soap, and then do a last, thorough rinse before drying the item. If the whole item is affected, just soak it in white vinegar, let it sit for a few hours (or overnight), and then rinse it thoroughly with water. If there are any stubborn part left, try scrubbing it with fine steel wool right after soaking it in white vinegar.
Rust removers are commercially available, and these solutions vary per application: some can be used on fabrics only, while some can be used on a certain type of metal only. One example of rust removers is WD-40.
Does WD 40 work?
WD-40 is a widely used rust remover. It does remove it; it diffuses through the porous iron oxide layer and expands to gas as it reacts with the material it is applied to. It can be used on iron, chrome, and stainless steel without compromising the metal underneath. However, it is important to do a test patch first since some metals get damaged upon usage of WD-40.
It has a multi-use-product spray variant, which is very easy to use. Simply spray it on the affected surface and scrub it with a wire brush. For items with grooves that are hard to reach and stubborn parts, you can opt for the WD-40 Specialist Penetrant that penetrates rust and grime better.
Be sure to also check out these articles on the comparison between gas & gasless welding, this one on Thermo King alarm codes, and this one that takes a closer look at the intricacies of welding. Meanwhile, you will want to make sure you’re choosing the right wire whether you’re needing 50 amp service or 200 amp service.