This is usually not caused by a single factor, but rather the result of a combined effect of the cleaning process chain, material properties and environmental factors. The fundamental reason is that the cleaning process “activates” the metal surface, leaving it in a more reactive and easily corrodible state, but subsequent protective measures fail to keep up.

The following is a systematic analysis of causes and solutions:

I. Core Cause Analysis (Step-by-Step Troubleshooting of the Cleaning Process)

Problem Stage Specific Cause Consequence (Rust/Degradation)

1. Cleaning Medium & Chemical Residue Improper cleaning agent selection: Using acidic, strongly alkaline, or high chlorine/sulfur content cleaning agents on sensitive metals (e.g., carbon steel, cast iron, copper alloys). Direct chemical corrosion, or leaving aggressive ionic residues on the surface.

 Cleaning agent concentration/temperature too high: Over-cleaning, destroying the metal surface passivation layer (e.g., the chromium oxide layer on stainless steel). Loss of the protective layer, exposing the metal substrate, making it susceptible to corrosion.

 Cleaning agent cross-contamination: Cleaning fluids from different processes contaminating each other, such as carrying a chlorine-containing degreaser into a subsequent process. Introduction of unpredictable corrosive factors.

2. Incomplete Rinsing (Most Common Cause) Poor rinsing water quality: Rinsing with tap water or hard water, which contains chloride ions, sulfate ions, or calcium/magnesium ions. Chloride ions easily cause pitting on stainless steel; calcium and magnesium ions form scale upon evaporation, promoting crevice corrosion underneath.

 Insufficient rinsing frequency/flow: Failure to completely displace residual cleaning agents from component crevices and surfaces. Corrosive chemical substances remain, continuously acting in a humid environment.

 Incorrect rinsing sequence: Failure to use multi-stage countercurrent rinsing (dirtiest parts enter the dirtiest rinse water, finally entering the cleanest rinse water), leading to contamination of the final water quality. Components carry out harmful ions upon completion.

3. Incomplete Drying (Key Cause) Improper drying method: Only using compressed air blow-drying or hot air drying, which is ineffective for blind holes, deep holes, or complex threads. Moisture residue remains, forming an electrolyte environment for a “corrosion cell.”

 Failure to use vacuum/steam drying: Unable to effectively remove the water film adsorbed on the surface. Microscopic moisture residue creates conditions for electrochemical corrosion.

 Post-drying cooling condensation: High-temperature workpieces are placed in a humid environment and cool rapidly, causing water droplets to condense on the surface. Directly leads to rapid rusting.

4. Impact of the Cleaning Process Itself Ultrasonic “over-cleaning”: Prolonged, high-power, especially low-frequency ultrasonic cleaning, can damage the metal's microscopic protective layer or cause cavitation corrosion. Creates microscopic defects, increases surface area, making corrosion more likely.

 Mixing different metals during cleaning: Placing metals with significantly different electrochemical potentials (e.g., copper and aluminum) in the same tank, forming a galvanic cell. The less noble metal (anode) undergoes accelerated corrosion (galvanic corrosion).

5. Post-Treatment & Storage Issues Lack of anti-rust step: Failure to apply anti-rust treatment (e.g., rust-preventive oil, VCI packaging) after cleaning and drying, according to storage requirements. Metal is directly exposed to humid air.

 Poor storage environment: High humidity, or a corrosive atmosphere (e.g., salt spray near the sea, acid/alkaline gases from chemical plants) in the warehouse. Environmental corrosive media directly attack the components.

 Personnel contact: Touching dried workpieces with bare hands; sweat (containing salts) on the hands leaves fingerprint-shaped corrosion marks on the surface. Introduces electrolyte and impurities.

II. Analysis of “Degradation” Phenomena for Different Metals

· Steel Components (Carbon steel, alloy steel): Mainly manifests as brown-yellow rust ($\text{Fe}_2\text{O}_3$). The root cause is the oxidation of iron.

· Stainless Steel: Mainly appears as punctate brown rust spots (pitting corrosion). The culprit is usually chloride ion residue, which locally destroys the passive film.

· Aluminum Alloys: Primarily shows white or grayish-white powdery corrosion products ($\text{Al}_2\text{O}_3$, etc.), or local blackening. May be related to cleaning agents being too strongly alkaline or contact with dissimilar metals.

· Copper and Copper Alloys: Mainly manifests as green corrosion (basic copper carbonate, i.e., verdigris) or red cuprous oxide spots. Usually caused by contact with sulfur/oxygen-containing moisture and air.