BUY CORROSION RESISTANT METALS

Corrosion is the loss of metal due to a reaction with the local environment. It is measured as the percentage of weight loss, or as the penetration rate of the corrosion, defined as Mils penetration per year (MPY) which is a unit of measurement equal to one thousandth of an inch.

Corrosion occurs in the presence of liquid or gas. It can happen at any temperature, however generally the corrosion rate increases as temperature increases. Liquid induced corrosion is often caused by impurities or by trace elements present in the liquid. For example traces of chlorine would encourage hydrochloric acid formation, or traces of sulfur would form sulfuric acid.

Corrosion resistant metals come in many forms. High temperature, corrosion resistant alloys are typically mixtures of various metals, including stainless steel, iron, nickel, chrome, copper, cobalt, tungsten, molybdenum and titanium, which can resist high temperature and corrosive environments more effectively than standard carbon steel.

It is important to remember that any alloy, including stainless steel, can corrode under certain conditions, so using the correct selection process for proper use of metals for a particular application is important.

Metals, especially stainless steels, form a very thin chromium oxide barrier layer which protects the metal from oxygen. Oxygen is needed to combine with iron to create rust or iron oxide. Generally, no oxygen means no rust. This layer forming is known as passivation and self-healing. For example if the surface is scratched, the oxide layer generates again in the presence of oxygen.

Common Types of Corrosion

There are many types of corrosion and some common classifications are listed below:

• Uniform Corrosion – Corrosion that forms evenly across all surfaces.
• Pitting Corrosion – Local corrosion that creates pits or cavities.
• Crevice Corrosion – Local attacks in crevices or concealed areas.
• Galvanic Corrosion – Local corrosion when materials exchange ions.
• Stress Corrosion Cracking – Where cracks happen for a specific corrosive environment.
• Inter Granular Corrosion – Local corrosion at the metal’s grain boundaries.
• High Temperature corrosion – This may take many different forms including:
  – Oxidation – The natural process where metal is converted to an oxide.
  – Metal Dusting – In the case of a high carbon environment which turns the metal to powder.
  – Ash or Salt Deposit Corrosion – This occurs in certain industrial processes when salt or ash are deposited and react with the alloy’s protective oxide layer.
  – Molten Salt Corrosion – Molten salts erode the oxide layer making the metal vulnerable to other corrosion types.
  – Sulfidation – Happens in the presence of naturally occurring sulfur compounds found in crude oil.
  – Carburization – A severe metal dusting condition which petrochemical components are designed to resist.
• Microbial corrosion – This caused by the aerobic or anaerobic metabolic activity of micro-organisms.
• Atmospheric corrosion – This is the most common type of corrosion (rusty iron) and is a result of the presence of oxygen and water in liquid or vapor form.

The Criteria for Selecting Corrosion Resistant Alloys

Corrosion resistance can be defined as the ability of a metal to prevent environmental deterioration by chemical or electro-chemical reaction. Corrosion resistant alloys therefore, have high resistance to reactions within the specific environment.

Corrosion resistant metals may possess one or more of the following attributes:

• Resistance to enhanced corrosion due to residual stress, applied stress, or fluctuating stress.
• High resistance to local attack, including localised pitting deep penetration, local stress corrosion cracks, and intra-granular corrosion.
• Minimum dissolution of the metal in aggressive solutions.
• Resistance to enhanced corrosion at the contact point under the load of two contacting and slipping surfaces.
• Resistance to the combined action of different corrosion sources.
• Resistance to selective dissolution of a more active constituent of an alloy, leaving behind a weak deposit of the other material – for example the dezincification of brass.
• Resistance to accelerated local corrosion where mating component assembly surfaces meet the corrosive environment.

Following are common families of corrosion-resistant alloys:

Nickel based
Nickel based alloys are extensively used in industrial applications partially due to their excellent corrosion resistant properties. They are resistant to fresh water corrosion in normal atmosphere, resistant to non-oxidizing acids and resistant to caustic alkalis. Additionally, they also perform well in high and low temperature, high stress environments.

Copper based alloys
Copper based alloys have excellent resistance to air, water, salt water, and many organic and inorganic chemicals. Combined with excellent thermal and electrical conductivity, high performance mechanical properties, and ease of machining ensures that these alloys are used widespread in the electrical industry. However there are some compounds and acids that aggressively attack these materials.

Stainless Steels
The most common of the corrosion resistant alloys, stainless steel, contains a minimum of 10.5% chromium. The chromium creates a self-healing oxide layer that generates the corrosion barrier. Combined with their other many performance and cosmetic features stainless steels have distinct advantages over other ferrous alloys. There are many different variations of the alloy designed according to the application.

For more information on how Future Alloys corrosion resistant special metals can suit your application, contact one of our material specialists.