Anticorrosive phosphating treatment process

Basic principles and classification of phosphating. Phosphating is a process in which chemical and electrochemical reactions are used to form a phosphate-based chemical conversion coating; the resulting phosphate conversion coating is known as a phosphated coating. The main purpose of phosphating is to provide protection for the base metal, thereby preventing corrosion of the metal to a certain extent ； Used as a primer before painting to improve the adhesion and corrosion resistance of the paint coat ； It is used as an anti-friction lubricant in metal cold working processes. 1 Basic principles The phosphating process involves chemical and electrochemical reactions. The phosphating reaction mechanisms for different phosphating systems and different materials are complex. Although scientists have conducted extensive research in this area, it has not yet been fully understood. A long time ago, the mechanism of phosphating to form a film was simply described by a chemical reaction equation: 8Fe + 5Me(H2PO4)2 + 8H2O + H3PO4 → Me2Fe(PO4)2•4H2O (film) + Me3(PO4)•4H2O (film) + 7FeHPO4 (sediment) + 8H2↑. Here, Me represents elements such as Mn and Zn. Machu and others believed that when steel is immersed in a high-temperature solution containing phosphoric acid and hydrogen phosphate salts, a granular phosphating film composed of phosphate precipitates is formed, along with iron monohydrogen phosphate sediment and hydrogen gas. This mechanical explanation is rather rough and cannot fully account for the film formation process. As research on phosphating progresses, the view widely accepted by scholars today is that the film-forming process of phosphating consists mainly of the following 4 steps: ① Acid etching reduces the H+ concentration on the surface of the base metal: Fe – 2e→ Fe2+; 2H2－+2e→2H2 (1). ② A catalyst (oxidizing agent) accelerates the reaction: Fe2+ + → Fe3+ + H2. Here, the catalyst (oxidizing agent) represents the oxidizing agent, while the reduced product represents Fe3+. By oxidizing the hydrogen atoms produced in the first step, the catalyst speeds up reaction (1), thereby causing a sharp decrease in the H+ concentration on the metal surface. At the same time, it also oxidizes Fe2+ in the solution to Fe3+. ③ The multi-step dissociation of phosphate: H3PO4 → H2PO4– + H+; H2PO4– → HPO42– + 2H+; HPO42– → PO43– + 3H+. (3) Due to the sharp decrease in the H+ concentration on the metal surface, the equilibrium of each step of phosphate dissociation shifts to the right, resulting ultimately in PO43–. ④ Phosphate precipitates and crystallize to form a phosphating film. When the PO43‑ ions released from the metal surface reach the solubility product constant Ksp with the metal ions in the solution (at the metal interface), such as Zn2+, Mn2+, Ca2+, Fe2+, phosphate precipitates are formed: Zn2+ + Fe2+ + PO43‑ + H2O → Zn2Fe(PO4)2•4H2O↓ (4); 3Zn2+ + 2PO43‑ + 4H2O = Zn3(PO4)2•4H2O↓ (5). The phosphate precipitates, together with water molecules, form phosphating nuclei, which then continue to grow into phosphating grains. Numerous such grains aggregate closely together to form the phosphating film. The side reaction of phosphate precipitation results in the formation of phosphide sludge: Fe3+ + PO43– = FePO4 (6). The mechanism described above can not only explain the film-forming process of zinc-based, manganese-based, and zinc-calcium-based phosphatings, but also guide the design of phosphating formulations and processes. From the above mechanism, it can be seen that an appropriate oxidant can increase the rate of reaction (2) ； A lower H+ concentration can shift the dissociation equilibrium of the phosphate dissociation reaction (3) to the right more easily, resulting in the release of PO43- ； When there is a combination of active sites and surfaces on the metal surface, precipitation reactions (4) and (5) can form phosphate precipitation nuclei without requiring a high degree of supersaturation ； The formation of phosphating sediment depends on reaction (1) and reaction (2); a high H+ concentration in the solution, as well as a strong accelerator, both lead to an increase in sediment. Accordingly, in actual phosphating formulations and process implementation, the surface features an appropriately strong accelerator (oxidizer) ； Higher acid ratio (relatively lower free acid, i.e., H+ concentration) ； Adjusting the metal surface to have active sites can increase the rate of phosphating reaction, enabling rapid film formation at lower temperatures. Therefore, when designing formulations for rapid phosphating at low temperatures, the aforementioned mechanisms are generally followed, involving the use of strong accelerators, a high acid ratio, and surface treatment processes. Regarding phosphating sludge. Since the phosphating sludge is mainly composed of FePO4, to reduce the amount of sludge it is necessary to decrease the production of Fe3+. This can be achieved in two ways: by reducing the H+ concentration in the phosphating solution (low free acidity) to minimize the oxidation of Fe2+ to Fe3+. The phosphating mechanism for zinc and aluminum materials is essentially the same as above. Zinc materials undergo phosphating at a relatively fast rate; the phosphating film consists solely of zinc phosphate, with very little sediment. Phosphating of aluminum typically requires the addition of large amounts of fluorine compounds to form AlF3 and AlF63-, and the process of phosphating aluminum follows a mechanism similar to the one described above. 2 Phosphating Classification: There are many ways to classify phosphating, but it is generally done based on the phosphating film-forming system, the thickness of the phosphating film, the operating temperature for phosphating, and the type of accelerator used. 2.1 Classification by phosphating film system: Based on the phosphating film formation system, they are mainly divided into six categories: zinc-based, zinc-calcium-based, zinc-manganese-based, manganese-based, iron-based, and amorphous iron-based. The main components of zinc-based phosphating bath solutions are: Zn2+, H2PO3-, NO3-, H3PO4, accelerators, etc. Main components of the formed phosphating film (steel parts): Zn3(po4)2•4H2O, Zn2Fe(PO4)2•4H2O. The phosphating grains are dendritic and acicular, with many pores. It is widely used as a primer before painting, for corrosion protection, and as an anti-friction lubricant in cold working. The main components of zinc-calcium-based phosphating bath solutions are: Zn2+, Ca2+, NO3-, H2PO4-, H3PO4, and other additives. The main components that form the phosphating film (on steel parts): Zn2Ca(PO4)2•4H2O, Zn2Fe(PO4)2•4H2O, Zn3(PO4)2•4H2O. The phosphating grains are in a compact granular form (sometimes with large needle-like grains), with few pores. Used for priming before painting and for corrosion protection. The main components of zinc-manganese-based phosphating bath solutions are: Zn2+, Mn2+, NO3-, H2PO4-, H3PO4, and various other additives. Main components of the phosphating film: Zn2Fe(PO4)2•4H2O, Zn3(PO4)2•4H2O, (Mn,Fe)5H2(PO4)4•4H2O; the phosphating crystals exhibit a mixed morphology of granular, acicular, and dendritic forms, with few pores. It is widely used as a pre-painting primer, for corrosion protection, and as an anti-friction lubricant in cold working. The main components of the manganese-based phosphating bath solution are: Mn2+, NO3-, H2PO4, H3PO4, and some other additives. The main component of the phosphating film formed on steel parts is: (Mn,Fe)5H2(PO4)4•4H2O. The phosphating film has a large thickness and few pores, with the phosphating grains appearing as dense particles. It is widely used in corrosion protection and as a anti-friction lubricant in cold working. The main components of iron-based phosphating bath solution are Fe2+, H2PO4, H3PO4, and some other additives. Main composition of the phosphating film (steel workpieces): Fe5H2(PO4)4•4H2O. The phosphating film has a large thickness, is formed at high temperatures, requires a long treatment time, contains many pores, and its crystalline grains are granular in shape. Used for corrosion prevention and anti-friction lubrication in cold working. The main components of the amorphous iron-based phosphating bath solution are Na+(NH4+), H2PO4, H3PO4, MoO4- (ClO3-, NO3-), along with some other additives. Main components of the phosphating film (for steel parts): Fe3(PO4)2•8H2O, Fe2O3. The phosphating film is thin, and its microstructural form consists of an amorphous phase arranged in a planar pattern; it is used solely as a primer before painting. 2.2 Classification by the thickness of the phosphating film: Based on the thickness of the phosphating film (i.e., its weight), it can be divided into four categories: sub-light grade, light grade, sub-heavy grade, and heavy grade. The next-lightweight type of coating has a weight of only 0.1–1.0 g/m2; it is generally an amorphous iron-based phosphating coating that is used solely as a primer before painting. It is particularly effective as a primer for workpieces that undergo significant deformation prior to painting. Lightweight films weigh 1.1–4.5 g/m2 and are widely used as primers before painting; they are less utilized in the corrosion prevention and cold working industries. The thickness of the phosphating film in the sub-heavyweight category is 4.6–7.5 g/m2. Due to its high weight and thickness (usually >3μm), it is less often used as a pre-coating base; it is only utilized as a base coat for steel parts that are not prone to deformation. It can be used for corrosion protection as well as for reducing friction during cold processing. Heavy-duty films with a weight of over 7.5 g/m2 are not used as primers before applying paint; they are widely employed for corrosion protection and cold working. 2.3 Classification by phosphating treatment temperature: Based on the treatment temperature, it can be divided into four categories: room temperature, low temperature, medium temperature, and high temperature. Room-temperature phosphating refers to phosphating without the use of heat. The typical treatment temperature for low-temperature phosphating is 30–45°C. Medium-temperature phosphating is generally at 60–70°C. High-temperature phosphating is generally above 80°C. The temperature classification method itself is not strict; sometimes sub-mesothermal and sub-hiperthermal classifications are also used, depending on individual preferences, but generally the aforementioned classification method is followed. 2.4 Classification by accelerator type: Since there are only a few types of phosphating accelerators, classifying them by type helps in understanding the bath composition. The phosphating temperature can generally be determined based on the type of accelerator; for example, NO3‑ accelerators are primarily used for medium-temperature phosphating. Accelerators are mainly classified into several types, including nitrate-type, nitrite-type, chlorate-type, organic nitrogen compound-type, molybdate-type, and others. Each type of accelerator can also be used in combination with other accelerators, resulting in numerous sub-series. The nitrate type includes: NO3‑ type, NO3‑/NO2‑ (spontaneous type). The chlorate types include: ClO3－, ClO3－/NO3－, ClO3－/NO2－. Nitrites include: nitroguanidine R-NO2-/ClO3-. The molybdate types include: MoO4‑, MoO4‑/ ClO3‑, MoO4‑/ NO3‑. There are many other classification methods for phosphating; for example, based on material, it can be divided into phosphating of steel parts, aluminum parts, zinc parts, and mixed parts.

How long is the stabilization time after phosphating?

Using phosphating as a corrosion protection method can only maintain corrosion resistance for a short period of time. Generally, phosphating is used only as a pretreatment step; anti-corrosion paint or similar substances must still be applied afterward.

Phosphating treatment is only effective for a short time and cannot last long

In a phosphate solution, heating and soaking are used to form a penetration layer of a certain thickness on the surface of the metal, thereby achieving corrosion prevention

Improving the adhesion and corrosion resistance of the paint film layer can enhance its resistance to cathodic delamination!

Generally, phosphating is not used as the final anti-corrosion measure; therefore, oil is applied or paint is sprayed after phosphating as the ultimate anti-corrosion protection. Some high-temperature phosphating processes still provide decent corrosion resistance, but they are not recommended.