Phosphating is the chemical process of forming a phosphate coating, a so-called conversion layer, on the surface of a metal such as steel or aluminium. The process plays a key role in industries where the strength and durability of materials are a priority, such as the automotive, aerospace and construction industries. In this article, we will take a closer look at the specifics of steel and aluminium phosphating, discussing its advantages, disadvantages and the technological nuances of the phosphating process.
What is phosphatisation?
Phosphating is the process of chemically transforming the surface of a metal using phosphate solutions. The chemical reaction between the metal and the solution leads to the formation of insoluble phosphate salts, forming a thin, protective coating. Depending on the chemical composition of the solutions used, there are three basic types of phosphating:
Iron phosphatising
Iron phosphating is a chemical process in which a thin layer of iron phosphate is formed on the surface of a metal (usually steel). It is formed by a chemical reaction between the metal surface and a phosphate solution containing the relevant chemical compounds. Iron phosphating is used primarily as a surface preparation method prior to the application of varnish or paint coatings, as it increases the adhesion of these layers and improves their durability.
The hallmark of this process is its versatility - it can be used in industries ranging from automotive to construction. The phosphate layer also acts as an anti-corrosion barrier, although its protective properties are less advanced than those of zinc phosphating. Due to its simplicity and lower cost, iron phosphating is often chosen for processes where the priority is to improve coating properties rather than maximum corrosion resistance.
Zinc phosphating
Zinc phosphating is a chemical process in which a layer of zinc phosphate is formed on the surface of a metal to protect it from corrosion and improve the adhesion of protective coatings such as paints or varnishes. The process involves immersing the metal in a solution containing phosphate compounds and zinc, which leads to a chemical reaction and the formation of a stable, porous coating. The zinc phosphate layer produced is thicker and more durable than that formed during iron phosphating, making it more suitable for harsh environments. For this reason, it is widely used in the automotive, construction or engineering industries, where components are exposed to moisture, salt or other aggressive substances. Although zinc phosphating is more costly than iron phosphating, its effectiveness in protecting the metal and extending the life of products makes it indispensable in many applications requiring the highest level of protection.
Manganese phosphating
During manganese phosphating, a layer of manganese phosphate is formed on the metal surface, which has a protective and anti-friction function. Manganese phosphating is distinguished by its exceptional resistance to mechanical wear, making it ideal for phosphating materials that require friction reduction and increased durability of components such as gears, bearings or firearm parts. The manganese coating has a greater thickness and hardness compared to other types of phosphating, such as zinc or iron. As a result, it offers better protection against mechanical damage and also increases corrosion resistance when combined with suitable lubricants. However, the process is more technologically demanding and expensive, making it mainly used in specialised industrial and military applications.
Read our article and reflections on the main disadvantages of phosphating -> https://www.kairospolska.pl/fosforanowanie-czy-to-najlepsza-metoda-przygotowania-powierzchni-przed-malowaniem-proszkowym/
Ferric phosphatising of steel and aluminium - fundamental differences
The process of phosphating steel is a widely used process in industry. During it, a reaction takes place between the surface of the metal and the phosphate solution, leading to the formation of a thin layer of iron phosphate, the so-called conversion layer. This layer is porous, which makes it ideal as a primer for paints or other protective coatings. In addition, it enhances the metal's resistance to corrosive agents, which is particularly important in aggressive industrial environments. Phosphating aluminium poses a greater technological challenge than steel, mainly due to the metal's lower chemical reactivity. The process requires specific chemical solutions adapted to the properties of aluminium. The resulting phosphate layer improves the adhesion of paint coatings, but its main role is to enhance corrosion resistance and minimise the risk of pitting corrosion.
Advantages of phosphating
Phosphating has been used extensively in a variety of industries over the years, demonstrating its versatility and effectiveness. The process is appreciated for its unique protective and preparation properties. The advantages of phosphating include functional aspects such as improving corrosion resistance or optimising the adhesion of paint coatings, making it a necessity in situations where the durability of materials is crucial. These features are discussed in detail below.
- Corrosion resistance - the phosphate layer effectively protects the metal from moisture and chemically aggressive substances.
- Improved adhesion - paintwork adheres better to the porous conversion layer produced during phosphatising, also improving its durability.
- Reduction of friction - in mechanical applications, manganese phosphating reduces component wear by lowering the coefficient of friction.
- Versatility - the process can be adapted to many types of metals, including stainless steel and aluminium.
Disadvantages of phosphatising
Despite its many advantages, phosphating also has its limitations, which are worth considering when planning the production process. These include both technological and environmental issues, which makes its use require adequate preparation and advanced knowledge. Understanding the potential disadvantages of phosphating allows the process to be optimised and the environmental impact and costs to be minimised.
- High energy intensity - The iron phosphating process requires a relatively large amount of energy, especially when heating the baths in special baths, which can increase production costs and negatively affect energy efficiency.
- Generation of chemical waste - Phosphating involves the use of chemicals and these, as hazardous waste, have to be disposed of properly. This presents an environmental challenge.
- Potential for corrosion after the process - If the phosphating process is not carried out or controlled correctly, there can be an excess or uneven layer of phosphate, which can lead to paint adhesion problems or corrosion in areas where the layer is not thick enough.
- Potential health risks - Although phosphatising is widely used in industry, the chemicals used in the process can pose health risks to workers, especially with improper handling or lack of proper protective measures.
- Unsustainability - Traditional iron phosphating generates chemical waste that can be difficult to completely dispose of or process. For this reason, the process is less environmentally friendly compared to modern, more sustainable technologies.
Phosphating steel and aluminium is an integral part of the surface preparation of metals in modern industry. However, it is important to bear in mind that the whole process has some drawbacks to contend with. Fortunately for the paint industry and manufacturing more widely, there are alternative methods for preparing metal surfaces for painting than phosphatisation.
