1. Project Background
In June 2026, a medical device company in the Dagang area of Zhenjiang urgently commissioned Blue Star Cleaning to process a batch of return transmission parts — totaling over 10,000 units. Each part consisted of an aluminum alloy precision-machined frame with a 45# carbon steel bushing heat-treated and press-fitted into place. The two materials could not be separated for individual treatment.
The parts had run for extended periods under high-temperature operating conditions. Bearing lubricant underwent oxidative polymerization catalyzed by heat and metal surfaces, forming a compact black oil-crust layer. Local thickness reached 0.5–1.5 mm, with strong adhesion to the substrate on some surfaces.
2. Cleaning Challenges: Aluminum vs. Alkali, Steel vs. Rust
2.1 Material Compatibility Conflict
Aluminum alloys are amphoteric metals — they corrode in alkaline environments above pH 9 and in acidic environments below pH 4. Carbon steel corrodes rapidly in acidic conditions and develops flash rust upon mere water contact. Conventional heavy-oil degreasers are typically strongly alkaline (pH 12–14), which would severely attack the aluminum. Neutral cleaners, on the other hand, struggle to dissolve deeply carbonized, hardened oil deposits. Reconciling these conflicting material requirements was the central challenge of this project.
2.2 Complex, Strongly Adherent Oil Deposits
After prolonged high-temperature service, the lubricant's base oil cracked and polymerized, additives decomposed, and metal wear particles combined to form a composite deposit of carbides, gums, asphaltenes, and metal oxides. The crust contained both non-polar components (base oil decomposition products) and polar components (oxidized polymers). No single solvent could effectively handle both fractions.
2.3 Difficult-to-Reach Dissimilar-Metal Interfaces
The press-fit interface between the aluminum frame and steel bushing contained micron-scale gaps. Oil had penetrated these gaps through capillary action, where conventional spraying or wiping could not reach. Deep interface cleaning required solvent penetration into the interfacial layer to detach oil deposits from both metal surfaces simultaneously.
2.4 Strict Wastewater Discharge Limits
The client's facility in the Zhenjiang New District operates under Grade I wastewater discharge standards (GB 8978-1996). Conventional high-pressure water cleaning consumes large volumes of water, and the resulting oil-laden wastewater exceeded the plant's on-site treatment capacity. Water conservation and source-level emission reduction were mandatory from process design onward.
3. Cleaning Solution: Neutral Solvent + Dual-Metal Corrosion Inhibition
3.1 Custom Heavy-Oil Solvent Formulation
To address the dual constraints — aluminum's intolerance of acids/alkalis and carbon steel's rust sensitivity — Blue Star Cleaning developed a pH-neutral, medium-to-high-efficiency heavy-oil cleaning solvent. The formula design principle: achieve effective degreasing within the pH 6.5–7.5 range while providing corrosion protection for both aluminum and carbon steel.
| Component | Function |
|---|---|
| Low-boiling-point solvent | Primary solvent — dissolves non-polar fractions in carbonized oil deposits |
| Polar co-solvent (glycol ethers) | Dissolves polar components — oxidized polymers, gums |
| Non-ionic penetrant | Lowers surface tension below 28 mN/m; penetrates the Al–steel interface micro-gap |
| Aluminum-specific inhibitor (silicate + organic phosphonate) | Forms a protective film on aluminum to suppress alkaline hydrolysis corrosion |
| Carbon steel inhibitor (organic amines) | Adsorbs onto steel surfaces forming a hydrophobic film; prevents rust during soak and rinse |
| Emulsifying dispersant | Keeps detached oil suspended and dispersed; prevents re-deposition on aluminum and steel |
3.2 Soak Process Parameters
Soak tank: 200 L · Batch size: 40–50 parts · Temperature: ambient
Soak time: ~10 minutes · Solvent reused throughout the day; replenished when effectiveness declines.
3.3 High-Pressure Water Fine Rinse
After soaking, the oil deposits were fully softened and loosened. Only brief high-pressure water rinsing was needed to remove residue. Two critical control points: the steel bushing grooves and the Al–steel interface roots must be thoroughly flushed, and the carbon steel bushings must not rust while still wet.
Rinse pressure: 18 MPa · Nozzle: 25° fan · Rinse time per part: 30–60 seconds
Post-rinse: drain and air-dry immediately (aluminum and steel surfaces simultaneously)
Total wastewater: ~40 L/batch · Over 80% reduction vs. full-pressure cleaning
4. Implementation Process
Two custom ~100 L stainless-steel mesh baskets were fabricated on steel stands to enable a dual-basket continuous-flow workflow.
Cleaning workflow: The first batch of 40–50 parts was loaded into a basket and submerged in the cleaning solution for approximately 10 minutes, with the liquid level at least 5 cm above the parts. After removal, high-pressure water rinsing was applied. Simultaneously, the second basket of 40–50 parts was loaded into the solution for its 10-minute soak. The two baskets alternated in a continuous cycle, maintaining steady throughput.
High-pressure rinse: A fan-nozzle water jet at 18 MPa was directed at each part, with special attention to the steel bushing grooves and the Al–steel interface roots. Each part received 30–60 seconds of rinsing, followed by a visual check — no visible oil residue remained.
Drain and dry: Rinsed parts were drained naturally and air-dried, with both aluminum and steel surfaces processed in sync. Throughput exceeded 3,000 parts per day. The solvent was checked daily for effectiveness and replenished or replaced when its cleaning power declined.
5. Cleaning Results
| Inspection Item | Standard Requirement | Result |
|---|---|---|
| Aluminum frame surface cleanliness | No visible oil residue | ✅ Pass |
| Carbon steel bushing cleanliness | No oil residue, no rust spots | ✅ Pass |
| Aluminum frame corrosion | No white spots, no pitting | ✅ Pass |
| Wastewater COD / volume | Daily wastewater <5,000 L | ✅ Pass |
Pass rate exceeded 90%. The client specifically praised two outcomes: the aluminum alloy surfaces were completely undamaged, and the carbon steel bushings showed zero rust marks after rinsing.
6. Process Advantages
Dissimilar-metal compatibility — dual protection for aluminum and steel. The neutral-pH formulation with an aluminum/steel dual-inhibitor system achieves efficient degreasing while protecting both materials. No white-spot corrosion on aluminum, no rust on carbon steel — solving the key industry challenge of excessive wastewater from conventional cleaning agents.
Interfacial penetration — deep degreasing. The low-surface-tension penetrant (<28 mN/m) carries solvent into micron-scale press-fit gaps between the aluminum frame and steel bushing, detaching oil from the interface layer — eliminating the blind spot of dissimilar-metal contact surfaces.
Source-level water conservation — environmentally responsible. The primary degreasing step is accomplished by solvent soak; high-pressure water serves only as a final rinse. Wastewater output is ~40 L per batch (under 20% of full-pressure methods), and the solvent is replenished on demand rather than discarded daily.
7. Conclusion
The core challenge in this case was not "getting it clean" — it was "getting it clean without damaging either of the two materials." Aluminum + carbon steel dissimilar-metal assemblies are extremely common in industrial equipment — bearing housings, transmission brackets, hydraulic valve bodies, and more — and every cleaning job for such parts is a compatibility balancing act between the two metals. Blue Star Cleaning's custom neutral dual-inhibitor solvent and low-pressure rinse process provides a proven solution for these mixed-material cleaning challenges.
The same process is applicable to aluminum-steel combination molds, jigs, pump bodies, valve bodies, and other precision parts requiring heavy-oil removal. Formulation ratios and soak parameters can be flexibly adjusted based on specific materials and deposit conditions. Contact us for a consultation: +86 18952832843.
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