Fighting Metal Corrosion: Phosphating

Corrosion is an electrochemical phenomenon that occurs on metallic materials, causing their deterioration. The phenomenon is generally caused by the interaction between the material and the external environment, but corrosion can be classified into different types depending on the type of material and the reactant.

However, a solution that can slow down and/or prevent the electrochemical phenomenon is phosphating. In the following paragraphs, we will see in detail what it entails.

How Does Metal Corrosion Occur?

A metallic material in contact with the external environment can corrode over time, thus becoming damaged. The gradual deterioration is caused by a chemical and/or electrochemical reaction with the environment, leading to rusting of the product.

Chemical corrosion occurs when the metal comes into contact with one or more chemical substances, while electrochemical corrosion occurs when the material comes into contact with specific chemical substances, in the presence of water or steam.

This second category includes different types of corrosion as follows:

Galvanic corrosion, contact between metals with different compositions (e.g., zinc, iron, and nickel);
Differential aeration corrosion, caused by a different concentration of oxygen in contact with the material;
Crevice corrosion, occurs when small crevices are present between two metallic areas or between a metallic and a non-metallic area;
Intergranular corrosion, occurs, for example, at the edges of the material where the lattice has been subjected to thermal activities (such as welding), leading to the actual crumbling of the piece;
Pitting corrosion, is an internal corrosive attack on the piece that causes perforations;
Stress corrosion, occurs when a material is simultaneously subjected to a tensile mechanical stress and a corrosive chemical attack.

Corrosion is a significant problem for metals. Indeed, the corrosive attack not only damages the piece but reduces the efficiency of the product, which must necessarily be replaced.

For this reason, it is essential to subject metals to protective treatments that can extend their life and, consequently, their effectiveness. Phosphating treatment is one of these.

What Phosphating Is For

Metal phosphating is a chemical pre-treatment that, applied to products, ensures full protection from external agents and extends their duration.

Technically speaking, phosphating can be explained as a chemical process through which a layer of zinc is deposited on the surface of a metal. Thus, a layer consisting of insoluble phosphates forms on the product, changing the chemical composition of the metal and, therefore, protecting it from the surrounding environment.

A greater seal and a better result are also guaranteed by a treatment that precedes phosphating and prepares the surface to be coated.

Before starting, the pieces must be perfectly cleaned, and two processes are performed:

Alkaline degreasing, to remove grease, oils, and other impurities;
Activation, the metallic surface is "activated" so that the phosphatic layer deposition is homogeneous.

Once the pieces are cleaned, the actual treatment begins. There are three types of phosphating:

Zinc phosphating, protection for iron, cast iron, and carbon steels according to the UNI ISO 9717 standard. The resulting color can be light gray or dark gray, while the thickness can vary within certain ranges established by the regulations. The treatment concludes with oiling and, based on requests, with dry drying.
Manganese phosphating, chemical treatment for lubricating iron, cast iron, and carbon steels that increases the adhesion of paints and protection from rust. Here too, the color varies from light to dark gray, and the thickness will change within certain ranges set by the regulations. The final processes of oiling and dry drying provide the piece with better lubricating action.
Black phosphating, treatment that has the same characteristics as manganese phosphating but is applied to achieve a darker and more uniform coloration, based on customer requests. An example is the drywall screws present in the catalog and subjected to this treatment.

Phosphated Products

Phosphated products find wide application in contexts where environments or corrosive agents can impact the painting and thus the durability of the coating. For this reason, this protection mechanism is widely used in the industrial and construction sector.

In general, the choice of phosphated products also depends on their final destination. Indeed, it is important to underline that based on their application, it is advisable to evaluate pieces subjected to phosphating or galvanizing.

In the latter case, these are products destined for harsh conditions (e.g., proximity to the sea, snow, heavy rains) and thus useful for structural manufactures for constructions. Their resistance to corrosive environments will surely be stronger compared to phosphated products.

Thus, the choice of protective treatment will be conditioned by the product's application.

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