1. Degreasing
The purpose of degreasing is to remove oil, grease, and other contaminants from the surface of the workpiece. However, residual ions such as Cl⁻ and SO₄²⁻ from pickling can cause serious post-corrosion damage to the workpiece.
Below is an introduction to chemical degreasing processes. In particular, phosphating prior to painting requires surface conditioning so that the metal surface attains a certain level of "activity." This ensures the formation of a uniform, fine, and dense phosphating film, improving paint adhesion and corrosion resistance.
Acidic cleaning agents can be used at low to medium temperatures.
2. Process Flow of Pickling and Phosphating for Steel Pipes
2.1 Pre-treatment Before Phosphating
In general, phosphating requires the workpiece surface to be clean bare metal (except for combined processes such as 2-in-1, 3-in-1, or 4-in-1 treatments).
Pre-treatment is the foundation for obtaining a high-quality phosphating film. It includes removal of grease, rust, oxide scale, and surface conditioning.
Methods are divided into two categories:
Mechanical methods: manual brushing, sandblasting, shot blasting, flame cleaning, etc.
Chemical methods: solvent cleaning, acidic cleaning, strong alkaline cleaning, and low-alkaline cleaning.
3. Degreasing Methods
3.1 Solvent Cleaning
Solvent degreasing typically uses non-flammable halogenated hydrocarbons via vapor or emulsion methods.
Common solvents include trichloroethane, trichloroethylene, and perchloroethylene.
Advantages: fast, highly efficient, and thorough removal of oils and grease.
Disadvantages: toxicity, high vaporization temperatures, and environmental concerns.
Due to the development of new water-based low-alkaline cleaners, solvent-based methods are now rarely used.
3.2 Acidic Cleaning
Acidic cleaning is one of the most widely used methods.
Common acids: hydrochloric acid and sulfuric acid (low cost, high efficiency).
Composition: surfactants (e.g., nonionic OP-type, anionic sulfonates), inorganic acids, and corrosion inhibitors.
Mechanism: utilizes emulsification, wetting, and penetration of surfactants, combined with hydrogen evolution from acid-metal reactions to mechanically strip off grease.
Can remove both rust and grease, often referred to as a "two-in-one" treatment solution.
Phosphoric acid-based cleaners do not leave harmful residues but are more expensive and less efficient.
3.3 Strong Alkaline Cleaning
Strong alkaline cleaning is a traditional and effective degreasing method.
Works by saponifying vegetable oils into water-soluble soaps.
Pure alkaline solutions cannot effectively remove mineral oils.
To improve performance, surfactants (typically anionic sulfonates) are added to enhance emulsification.
Note:
Acidic cleaners are generally not suitable for zinc and aluminum parts, especially zinc, which corrodes rapidly in acid.
4. Temperature Considerations
Low temperature: removes mainly liquid oils
Medium temperature: removes both oils and grease
Typically suitable for immersion processes
5. Summary
Before phosphating, workpieces must undergo proper pre-treatment including degreasing, rust removal, scale removal, and surface conditioning. This ensures the formation of a high-quality phosphating film with strong adhesion and excellent corrosion resistance.