The natural corrosion resistance of metals and their alloys is one of the reasons for the popularity of aluminum or titanium. Resistance to the gradual degradation and damage of the surface of metal (and often – deeper layers) is important for many industries – from home appliances, and construction to automotive, airplane, and space. The scale of corrosion resistance is an important factor that should be taken into consideration during the process of selecting the right metal alloys for parts manufacturing.
What is metal corrosion resistance?
By the book – metal resistance to corrosion can be defined as 'maintaining a thermodynamic balance between the environment and the metal surface'. In other words – does the environment around the metal part not cause noticeable oxidation, that can lead to damaging its surface? Metal alloys have natural corrosion resistance – among the most popular in many industries it is worth mentioning aluminum, copper, or titanium. The surface of these materials can oxidize but most often the contact with the corrosive environment leads to the creation of a thin layer of oxide that can naturally protect deeper structures. The perfect example of that process is aluminum alloys.
On the other hand, one of the most popular processes of increasing metal corrosion resistance is a surface bettering, most often galvanic, that can provide satisfying protection against the elements.
Among these processes is worth mentioning titanium and aluminum anodization or surface saturation with elements (eg. the creation of a thin zinc layer in the galvanic processes).
The alternative to these processes remains painting (with lacquers and specialized paints) or coating the surface with proprietary anti-corrosion processes (eg. Cerakote). It is worth mentioning that noble metals have very high natural corrosion resistance on the whole surface. Especially objects made out of gold, platinum, or silver of the highest proof, with marginal additives. Due to the very high price of these materials (and suboptimal mechanical performance) – most of the industries use other types of alloys with additional processes.
Types of metal corrosion
Outside of the elite group of metals (that can be called – precisely – noble) – metal and its alloys corrode. If such a process takes place, the surface develops rust–oxides and hydroxides of metals (thus: oxidation) that are the most recognizable signs of such a degradation process.
Literature defines two most important types of corrosion due to its main agent:
- chemical corrosion – the effect of a chemical reaction between the metal surface and the environment (dry), without exchange of electrical potentials. The main agents of that process are active elements and chemical compounds – oxide (O2), carbon dioxide (CO2) or hydrogen chloride (HCl),
- electrochemical corrosion – the result of electrochemical processes that lead to (mentioned) potential difference. The effect of that process is metal oxidation and the development of loss in the affected area in the form of a layer of oxides and hydroxides. The best example of that process is the reaction between steel and water – metal is a conductor, its crystal structure is not unique, and the surface has different standard potentials. Water has some ion conductivity that can cause potential differences. This is an electrochemical description of something that most people describe as 'steel in contact with water will rust'. In fact – the process itself is much more complicated. Of course – some environments or agents can be more aggressive and lead to faster oxidation – eg. salt water, especially in warm areas with a higher level of salts, especially on metals that are subjective to the corrosion (eg. steel without additional galvanic coating).
The corrosion itself can be divided due to its effect on the metal surface for:
pitting corrosion – can create deep 'pits' of corrosion in the metal surface, especially in areas with structure more subjective to the anode-cathode reaction (in other words: ion conductivity). Such corrosion can lead to the development of very deep loss in metal surfaces,
concat corrosion – another name for the reaction that can take place between less and more noble material surfaces upon longer contact. A good example of that process is a reaction between bolts used to attach bigger metal elements. If one of the materials is made out of 'less noble' metal, it can become a corrosion inhibitor. What is even more interesting – such a process can be used to protect metal elements from corrosion. For example – in salt water pumping stations to protect steel pipes,
stress corrosion – appears on the weakened surface of many alloys. Rusted spots can appear on many types of damages caused by stress dispersion – from working to even created during manufacturing.It is worth mentioning that corrosion accelerators can be also agents of high temperature, mechanical, or chemical damage to the surface. That is why even small chips of the protecting layer of the car body can become corrosion spots, especially in time of sprinkling salt on the roads (salt can speed up metal oxidation processes).
Metals with natural resistance to corrosion
Corrosion-resistant alloys are among many:
- noble metals – platinum metals, copper metals,
- half-noble metals – although this is a more conventional category, that group includes metals with low reactivity. and are often used to saturate/coate other metal surfaces in galvanic processes. The best examples of that group are copper, chromium, nickel, and zink, which can stop the corrosion attack due to protection provided – for example – by chromium oxide,
- other metal of low reactivity, such as aluminum (often used in machining processes due to good mechanical parameters and good to very good corrosion resistance, series dependent) or titanium. It is worth mentioning that these metals can also corrode, but the rust looks different. It is similar to the white coating that is typical of 'red rust on steel'.
Other materials and alloys, such as stainless steel, can achieve better corrosion resistance in the process of adding other casting elements (eg. chromium). Other surface-bettering processes can improve metal parts' corrosion resistance. Among the most popular and effective it is worth mentioning:
- galvanic processes – chemical and electrochemical processes that are focused on developing thin layers of oxides on the surface of metal parts. Such a surface provides good corrosion resistance, depending on the process and the layer thickness (defined by the particular process). Examples of galvanic processes are anodization (aluminum, titanium) or zink coating,
- powder painting – often selected due to low cost and 'good enough' corrosion protection,
- traditional painting, lacquering – often used in the automotive industry, provides acceptable corrosion protection, but the mechanical resistance for the elements leaves some to be desired.
What is the resistance of metals to corrosion (on scale)
Many categorizations describe metal resistance to corrosion. In Poland, the most popular remains one presented in the 'Encyclopedia of Technology. Metallurgy' from 1978 that describes – so-called – general corrosion. The methodology behind that categorization based on metal corrosion in time (12 months) and is very useful to determine what is the natural corrosion resistance of a particular alloy.
1. Metals completely resistant to corrosion (group I), described as metal alloys that corrode >0,001 mm per year (level of resistance = 1),
2. Metals very resistant to corrosion (group II), described as metal alloys that corrode less than 0.01 mm per year (level of resistance = 2 to 3).
3. Metals that are resistant to corrosion (group III), are described as metal alloys that corrode between 0.01 and 0.1 mm per year (level of resistance = 4-5).
4. Less corrosion-resistant metals (group IV), are described as alloys that corrode between 0,1 and 1 mm per year (level of resistance = 6-7).
5. Lesser corrosion-resistant metals (group V), that corrode between 1 and 10 mm per year (level of resistance = 8 to 9),
6. Metals not resistant to corrosion (group VI) that corrode between 10 and more mm per year (level of resistance = 10).
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