Comparison of pre-treatment processes for spraying iron and aluminum parts

There are significant differences in the pre-treatment processes for iron and aluminum parts prior to coating due to differences in material properties (e.g., corrosion resistance, surface oxidation layer, chemical reaction activity, etc.). The following is a key comparison between the two:

1.Degreasing process

Iron parts:

Highlights: Removal of grease, rust preventive oils or processing residues.

Method: Alkaline degreasing (pH 10~12) or solvent cleaning.

Note: Avoid excessive alkalinity that may cause loosening of the subsequent phosphating film.

Aluminum parts:

Highlights: Remove grease and aluminum oxide film.

Method: Weak alkaline degreasing (pH 9~10) to avoid strong alkali corrosion of aluminum surface.

Note: Need to add corrosion inhibitor (e.g. silicate) to prevent excessive corrosion of aluminum.

2.Rust/oxide removal

Iron parts:

Process: Pickling (hydrochloric or sulfuric acid solution) to remove rust (Fe₂O₃) and welding slag.

Risk: Excessive pickling can lead to hydrogen embrittlement (control of acid concentration and time required).

Alternative: sandblasting or shot blasting (physical descaling, suitable for thick layers of rust).

Aluminum parts:

Process: Acid deoxidation (nitric acid/sulfuric acid mixture) or alkaline etching (NaOH solution) to remove the natural oxide layer (Al₂O₃).

Risk: Strong acid/alkali will easily lead to uncontrolled surface roughness, and the concentration and temperature should be strictly controlled.

3.Surface conversion layer treatment

Iron parts:

Mainstream process: phosphate treatment (zinc system, iron system or manganese system phosphate).

Function: Generate porous phosphate layer, improve coating adhesion and rust prevention ability.

Parameters: pH 2~4, temperature 35~55℃, time 5~15 minutes.

Aluminum parts:

Mainstream process:

• Chromating process: hexavalent chromium passivation (gradually phased out, environmental restrictions).
• Chromium-free passivation: zirconium/titanium conversion film (environmental trend).
• Anodizing: Generate dense aluminum oxide layer (Al₂O₃), suitable for high-end needs.

Function: Enhance corrosion resistance and provide coating bonding.

Parameters: pH 3~5 (chrome-free process), room temperature or low temperature treatment.

4.Water washing& surface conditioning

Iron parts:

Water washing: Need to thoroughly remove residual acid/alkali to prevent contamination of the phosphating solution.

Surface conditioning: titanium phosphate colloidal solution to refine the phosphating crystals.

Aluminum parts:

Water washing: deionized water is required to avoid calcium and magnesium ion residues affecting the passivation film.

Surface Conditioning: Usually no surface conditioning is required, some processes may use fluorinating agent to activate the surface.

5.Environmental protection and cost differentials

Iron parts:

Environmental protection: Pickling waste solution contains Fe²⁺/Fe³⁺, phosphating waste solution contains heavy metals (e.g. Zn²⁺).

Cost: Phosphatization cost is lower, but waste liquid treatment cost is higher.

Aluminum parts:

Environmental: Hexavalent chromium from chromating process is highly toxic and requires alternative technology; alkaline etching wastewater needs to be neutralized.

Cost:Chromium-free passivation/anodizing is costly, but meets environmental regulations.

6. Summary of key process differences

7.Application recommendation

Iron parts: Phosphating + sandblasting combination is recommended for heavy anti-corrosion scenarios (e.g. car chassis).

Aluminum parts: Chrome-free passivation or anodizing is recommended for lightweight and high appearance requirements (e.g. 3C electronics, aerospace).

The performance of the sprayed coating can be significantly improved by targeted pre-treatment, while the selection of the process needs to be made in the light of environmental regulations and costs.