Corrosion Protection

What is corrosion?

Corrosion is a natural, chemical or electrochemical process during which materials are corroded or decomposed due to reactions with their environment. These reactions cause a change in the physical and chemical characteristics of a material, which can ultimately impair its functions and structural integrity. Corrosion may take on many forms, including rusting of iron and steel, pitting, intergranular corrosion and stress corrosion cracking.

Common causes of corrosion

• Moisture and oxygen: one typical example of corrosion is rusting of iron and steel. When these metals are exposed to moisture and oxygen, they react and form iron oxide (rust), which is porous and fragile and destroys the metallic structure.
• Chemical substances: salts, lyes and acids can trigger chemical reactions which cause corrosion.
• Electrochemical reactions: these reactions often occur in metals which are joined to one another in an electrolyte (e.g. brine or salt water), which can cause galvanic corrosion.

Types of corrosion protection

Corrosion protection is essential to protect metal components against corrosion. Different types of protection are used with their type depending on the material which requires protection. We basically make a distinction between active and passive corrosion protection. Some methods combine both approaches, one example being hot-dip galvanisation.

Active corrosion protection

Active corrosion protection refers to methods which use targeted technical measures to achieve a direct impact on the corrosion process. There is no need for a complete separation between the material and the corrosive medium in such cases.

Examples of active corrosion protection include:

• Sacrificial anode: a sacrificial anode is made of a less noble metal which is purposely fitted to corrode, thus safeguarding the metal requiring protection against corrosion. Electrons flow from the sacrificial anode and thus prevent the main metal from rusting.
• Cathodic protection: in the case of cathodic protection, the metal requiring protection is safeguarded against corrosion by using sacrificial anodes, such as zinc or magnesium, which corrode instead of the metal, or by applying an electric current.
• Zinc flake coating: this anti-corrosion coating consists of thin layers of zinc and aluminium flakes bonded together in a binder matrix. They are applied to the metal and then hardened.
• Inhibitors: here, chemical substances slow down or impair the corrosion rate of metals in aggressive environments. They take effect by attaching themselves to the metal surface, forming a protective barrier, or by producing a chemically reaction to reduce the environment’s corrosive characteristics.
• Hot-dip galvanisation: hot-dip galvanisation is a corrosion protection method where iron or steel is dipped into molten zinc at around 450 °C and coated with a layer of zinc. This layer protects the metal from corrosion by forming a physical barrier and acting as a sacrificial anode.

Passive corrosion protection

Passive corrosion protection comprises attaching different substances to a material, thus providing a protective layer. The material is fully shielded from contact with corrosive media, protecting it from corrosive influences.

Examples of active corrosion protection include:

• Protective paints or protective coatings: special materials are applied to metal surfaces to protect them against corrosive influences. The coatings form a physical barrier between the metal and its environment, which keeps corrosion-inducing substances at bay.
• Wax, oils and greases: these substances create a protective barrier on the metal surface. They penetrate microscopically small cracks and pores in the metal, thus preventing direct contact with air and moisture, the causes of corrosion.
• Passivation: this technique consists of forming a stable passive layer on the metal surface to give protection against corrosion. This layer forms spontaneously on certain metals, such as stainless steel or aluminium, and reduces their reactivity to harsh environments.
• Metallic protection using metals such as zinc or aluminium: a protective layer of these materials is applied to the metal surface. It creates a physical barrier that protects the metal beneath against corrosion due to moisture, oxygen and other corrosive influences.
• Hot-dip galvanisation: hot-dip galvanisation is a corrosion protection method where iron or steel is dipped into molten zinc at around 450 °C and coated with a layer of zinc. This layer protects the metal from corrosion by forming a physical barrier and acting as a sacrificial anode.

Further approaches to corrosion protection

In practice, experts and companies often make a distinction between temporary and permanent corrosion protection.

Temporary corrosion protection

Temporary corrosion protection is designed to preserve components against corrosion for a limited time period, ranging from a few days to a maximum of two years. This type of protection is normally passive and is primarily used for transporting and storing components and workpieces. It is removed before components and workpieces are used or further processed. Oil or wax-based products are particularly suitable for such protection.

Permanent corrosion protection

Permanent corrosion protection is designed to preserve components against corrosion for a long period of time, often a number of years or decades. This protection consists of multiple layers built on top of one another. Typically, the first layer provides active corrosion protection with subsequent passive layers applied on top. Permanent corrosion protection cannot be removed once applied. Metal-based paint systems offering cathodic corrosion protection are eminently suitable as a first layer. Further coating layers can then be added as required.

Tests and industrial standards for corrosion protection

There are standards in place for implementing and evaluating corrosion protection measures.

• EN ISO 12944 provides definitions for environmental conditions and minimum corrosion protection requirements in different classes in relation to permanent corrosion protection for components.
• Section 6.3 in EN ISO 1461 specifies how to repair flaws and defects in hot-dip galvanised parts.
• One of the key standards for corrosion protection products is EN ISO 9227, which stipulates a salt spray test. This test serves as the basis for classifying corrosion protection as per EN ISO 12944.
• Paint-based corrosion protection products are also subject to further tests that take into account stress loads on coatings, such as bending or damage. Lubricants with special additives can protect metal surfaces from moisture and aggressive chemical compounds that may cause corrosion.