Corrosion By Ross James and Alex

Corrosion- What is it? Corrosion comes from the Latin word “Corrodere” and means “To gnaw”. Corrosion is the gradual destruction of most metals, this can be through a chemical or electrochemical reaction to its environment (oxidization with Air and water). The most common form of corrosion is oxidation. Can also be described as a transfer of electrons from one metal to another.

Corrosion- How it Happens? There are many different types of corrosion: Galvanic corrosion Pitting corrosion Crevice corrosion Microbial corrosion High temperature corrosion Passivation

Galvanic Corrosion Galvanic corrosion occurs when two dissimilar metals are in contact with each other or have an electrical current passing through them while they are both immersed in an electrolyte, (salt water). Because the metals are different, one will be more active (the Anode) and one will be more noble (the cathode). The Anode will be the metal that corrodes while the Cathode is protected. Cathodes have more electrons in their outer shell and so readily accept electrons while anodes have less and so are ready to give up electrons and therefore degrade. This type of corrosion is typically used to protect a metal by sacrificing the Anode, typically on Steel structures.

Galvanic Corrosion This is an example of galvanic corrosion between steel and aluminium. Aluminium is more active than the steel and so is perforated and the joint between the steel and aluminium.

Galvanic Series This is where the resistance to corrosion is determined, also known as ‘Nobility’. The test ‘Electrolysis’ is used to determine the Nobility of materials. Electrolysis is where an anode and cathode and submerged in an electrolyte solution. An electrolyte solution such as salt water is exploited used to accelerate the corrosion process. Both the anode and cathode are attached to an electrical current, the anode to the positive terminal and cathode the negative. This test is a controlled ‘Galvanic Corrosion’ process. Anodic is sometimes described as being ‘Active’ because it has high corrosion potential. Cathodic therefore is described as being ‘Less Active’ these materials have less corrosion potential, or Nobility. From wikipedia “When two metals are submerged in an electrolyte, while electrically connected, the less noble (base) will experience galvanic corrosion. The rate of corrosion is determined by the electrolyte and the difference in nobility. The difference can be measured as a difference in voltage potential. Galvanic reaction is the principle upon which batteries are based.”

Implications- Statue of Liberty Corrosion degrades the useful properties of materials and structures including strength and appearance. In the 1980’s regular maintenance shows that galvanic corrosion had taken place between the copper skin, and the iron support structure. Although this was anticipated and insulation ‘Shellac’ which was used between the two metals also failed over time, resulting in corrosion of the iron support structure. Renovation took place to replace the old insulation with PTFE. The structure itself although was partially corroded was largely unaffected and was therefore deemed safe enough to leave it.

Pitting Corrosion Pitting corrosion is a very localised form of corrosion, which causes small holes in the metal affected. Pitting corrosion is a very localised form of galvanic corrosion and the galvanic corrosion can be started by a small scratch or damage to a protective coating. Pitting is more common in alloys that are protected against corrosion by using a passivation layer. These include stainless steel, nickel alloys and aluminium alloys. Normal carbon steel will not tend to pit as mild steel corrodes uniformly.

Pitting- Example A single pit in a critical point can cause a great deal of damage. One example is the explosion in Guadalajara, Mexico on April 22, 1992, when Gasoline fumes accumulated in sewers destroyed kilometres of streets. The vapours originated from a leak of gasoline through a single hole formed by corrosion between a steel gasoline pipe and a galvanised steel water pipe.

Crevice Corrosion Crevice corrosion occurs in confined spaces where a gap is large enough to allow the ingress of corrodent however are small to allow it to stay stagnant. These could be small cracks and seams or contact areas between parts. Poor design can lead to crevice corrosion such as overlapping surfaces, under gaskets and around poorly welded areas. Crevice corrosion can be seen as a less severe type of localised corrosion when compared to pitting corrosion, in that the depth and rate of corrosion from pitting is much greater.

Microbial Corrosion This is also called bacterial corrosion and is caused by micro-organisims, the most common being chemoautotroph. Microbial corrosion can effect metallic and non metallic materials. Some of the bacteria produce chemical compounds, and acid iron oxides and iron hydroxides. Hydrocarbon utilizing microorganisms or “HUM bugs” can be a problem in jet fuels as they can cause corrosion to plastic and rubber parts of the fuel system. They also can corrode the metallic parts FSII is an additive which is mixed with the fuel, this slows the rate of growth of the micro-organisms.

High temperature corrosion High Temperature Corrosion – Is a type of corrosion which occurs in machinery which comes into contact with hot gases which contain contaminants. Fuel sometimes contain sulphates which melt at very low temperature during combustion. The liquid salts are extremely corrosive for stainless steel and other alloys which are usually resistant to corrosion and high temperatures. Typical there are two type of sulphate induced high temperature corrosion, type 1 and type 2. Type 1 takes place above the melting point of sodium sulphate. Type 2 takes places below the melting point of sodium sulphate, but in the presence of SO3.

Preventing Corrosion

Applying Protective Coatings Putting a barrier of material between the metal to be protected and the corroding environment. Painting Exposed metal can be primed and painted to stop air and water coming into contact with the metal. A break in the paint such as a scratch will expose the metal and corrosion will start. Electroplating Metals can be plated with other metals which provide corrosion resistance. Two metals are submerged in a solution called an electrolyte, a current is passed through which causes the anode to be dissolved and plated onto the cathode.

Electroplating Electroplating Materials Copper Gold – Excellent corrosion resistance Silver Nickel – Good corrosion resistance Tin – Good corrosion resistance Solder – Good corrosion resistance Brass Cadmium Palladium Zinc – Good corrosion resistance Some of these materials are not used to prevent corrosion but are still plated onto other metals due to their other properties such as conductivity.

Galvanising Galvanising is when a protective coating of zinc is applied to steel or iron to prevent rust. Hot-dip galvanisation is when the iron or steel to be coated is passed through a molten bath of zinc. Galvanisation can also be done using an electroplating method but the coating will not be as thick as the hot-dip method. If the zinc coating is scratched, the zinc will act as an anode and will still corrode before the steel or iron.

Galvanising Image from http://upload.wikimedia.org/wikipedia/commons/0/03/Galvanized_surface.jpg

Corrosion Inhibitors Corrosion inhibitors can be added to a liquid or gas to suppress their corrosive properties. Such as anti-freeze and water in the radiator and cooling system for an engine. This method for preventing corrosion is usually used in closed loop situations such as a cooling loop. Some chemicals that prevent corrosion are chromates, phosphates and polyaniline.

Passivation Passivation occurs naturally or can be applied with other methods. In air, most metals will form a natural hard layer of oxide or nitride. These layers are more noticeable on aluminium, zinc, titanium and copper. By forming an outer layer of oxide or nitride, corrosion is prevented from penetrating further.

Anodising Anodising is usually used for aluminium and is a method of passivation Uses electrolytic passivation to increase the thickness of the natural oxide layer. The aluminium acts as an anode in an electrolytic solution as a current is passed through it. After the electrolytic passivation process is complete the aluminium needs to be sealed with a dye which is why anodised components can come in many colours. If the layer is scratched normal corrosion will occur at that location. The layer is fairly resistant, but should be cleaned regularly to avoid panel edge staining.

Anodising Image from http://upload.wikimedia.org/wikipedia/commons/f/fd/Belaying8.jpg

Sacrificial Anodes (Passive Cathodic Protection) Sacrificial anodes are a passive method of protection and are usually used when submerged in water or another liquid that conducts electricity. A sacrificial metal is attached to the metal to be protected. The sacrificial metal acts as an anode and the metal to be protected acts as a cathode. The two metals must have good electrical contact. The sacrificial anode should have a more negative electrical potential than the cathode. Which is shown on the galvanic series. Since the cathode has more electrons in the outer shell than the anode, the cathode will accept electrons from the anode via electrical current, preventing it from corroding.

Sacrificial Anodes Image from http://upload.wikimedia.org/wikipedia/commons/3/38/Electrode_protecting_a_screw.jpg

Active Cathodic Protection Works on the same principle as passive cathodic protection The anode and cathode are connected to a low voltage DC power source. Active protection can protect a larger area than passive protection. The voltage provided by the power source can be adjusted to provide optimum protection.

Active Cathodic Protection

Preventing Corrosion by Design Corrosion tends to start in small crevices and confined spaces first. Careful consideration should be taken when designing a component that will be open to the elements. Older cars tend to be prone to corrosion where moisture can collect.

Any Questions?