Comprehensive Optimization of AZ31 Magnesium Alloy Semi-Synthetic Cutting Fluid

As the lightest structural metal, magnesium alloys are increasingly widely used in 3C electronics, automotive lightweighting, aerospace, and other fields, thanks to their excellent specific strength, electromagnetic shielding, and vibration damping. AZ31 is the most widely used wrought magnesium alloy grade, but its high chemical activity and low standard electrode potential of about −2.37 V make it highly susceptible to electrochemical corrosion during machining. The white-spot problem that appears on workpiece surfaces after cutting has long troubled manufacturers, directly affecting product appearance and yield.

This article proposes a comprehensive optimization solution for a semi-synthetic cutting fluid for AZ31 magnesium alloy, with eliminating white spots on the surface after cutting as the core objective, while also addressing foam control and wetting performance. The original formulation has a pH of 8.6–9, is used at a 10-fold dilution, and contains sodium petroleum sulfonate, Turkey red oil, triethanolamine, and other components.

1. Diagnosis of the White-Spot Problem

1.1 Nature and Causes of White Spots

Corrosion morphology	Characteristics	Link to existing formulation
Pitting-type white spots	Localized accumulation of white corrosion products	Insufficient compactness of the sodium petroleum sulfonate protective film, Cl⁻ penetration
Filiform corrosion	Linear white-spot propagation	Incomplete film, driven by oxygen concentration differences
Uniform loss of gloss	Overall whitening without gloss	Protective film too thin or insufficient adhesion

Core diagnosis: The white spots are most likely pitting-type corrosion—the protective film formed by sodium petroleum sulfonate on the magnesium alloy surface is incomplete or insufficiently compact, allowing Cl⁻ or other aggressive species to penetrate and trigger localized corrosion.

1.2 Key Optimization Directions

The original approach prioritized formulation compatibility; the optimized approach shifts to coordinated efforts across three dimensions: film compactness + uniformity + localized pH buffering.

2. Optimization Solution

2.1 Core Principles

• Keep the corrosion inhibitor unchanged: maintain Vanconol® MGR-83 as the primary corrosion inhibitor;
• Enhance protective film integrity: improve film quality through process optimization and additive supplementation;
• Add Toynol® additives: optimize auxiliary functions such as wetting and defoaming.

2.2 Component Function Positioning

Component	Role	Optimization direction
Vanconol® MGR-83	Primary corrosion inhibitor	pH matched to 9–10, high-temperature stable, organic hybrid type
Sodium petroleum sulfonate	Lubrication/emulsification auxiliary	No longer relied upon for corrosion inhibition
Triethanolamine	pH adjuster	Control total amine content ≤3%
Turkey red oil	Lubrication auxiliary	Retain original function
Toynol® additives	Functional supplementation	Improved wetting, defoaming, and stability

3. Recommended Toynol® Additives

3.1 Wetting Agent Selection

Product	Reason for recommendation	Dosage in concentrate
BWA-925	Low-foam performance with foam dissipating within 300 seconds, reducing foam-induced damage to protective film integrity; a dynamic surface tension of 43.8 mN/m promotes uniform spreading	0.05%–0.15%
FS-204E	Combines wetting and foam control in one, reducing total additive loading; a dynamic tension of 32.4 mN/m helps fill micro-defects	0.1%–0.2%

Selection advice: Since foam can cause an uneven protective film, prefer low-foaming wetting agents; uniform spreading helps form a continuous protective film.

3.2 Defoamer Selection

Product	Reason for recommendation	Dosage in concentrate
DF-80	Molecular defoamer that does not oil out, avoiding oil spots that affect film continuity; good compatibility with waterborne systems	0.05%–0.1%
Foamic-024	Dedicated for epoxy systems, stable under alkaline conditions; long-lasting foam suppression prevents foam regeneration during machining	0.05%–0.15%

Selection advice: Foam generated during machining can cause localized loss of the protective film, so select a stable, long-lasting defoamer combination.

3.3 Combination Options

Option	Core combination	Advantage for the white-spot problem
Option A	BWA-925 + DF-80 + Foamic-024	Wetting + staged defoaming, maximizing film integrity
Option B	FS-204E + Foamic-024	Two-in-one wetting/foam control + dedicated defoaming, reducing additive interactions

4. Complete Optimized Formulation

4.1 Formulation Framework

No.	Component	Recommended dosage (mass fraction)	Optimization notes
1	Deionized/softened water	Make up to 100%	Removes Cl⁻ interference; must be used
2	Triethanolamine	≤3%	Control total amine content
3	Sodium petroleum sulfonate	≤5%	Downgraded to a lubrication auxiliary
4	Turkey red oil	≤3%	Retain original function
5	Vanconol® MGR-83	0.5%–1.0%	Primary corrosion inhibitor
6	Toynol® BWA-925	0.05%–0.15%	Low-foam wetting, ensuring uniform spreading
7	Toynol® DF-80	0.05%–0.1%	Molecular defoaming, avoiding oil-spot effects
8	Toynol® Foamic-024	0.05%–0.15%	Long-lasting foam suppression, preventing foam regeneration during machining
9	Borate buffer	0.3%–0.5%	Enhances localized pH buffering
10	Silicate	0.1%–0.2%	Optional addition, fills micro-defects in the film
11	Other defoamers	As needed	—
12	Biocide	As needed	—

4.2 Summary of Key Adjustments

• Water quality requirement: deionized/softened water must be used to remove the Cl⁻ aggressive factor;
• Buffering reinforcement: add a borate buffer to stabilize the localized pH;
• Film enhancement: optional silicate to fill micro-defects in the organic film;
• Additive optimization: Toynol® additives emphasize low foam and uniform spreading to ensure protective film integrity.

5. Optimized Preparation Process

1. Heat deionized/softened water to 40–50℃; tap water is strictly prohibited;
2. Add triethanolamine and stir for 5–10 min;
3. Add sodium petroleum sulfonate and Turkey red oil, and stir for 30 min until uniform;
4. Add the borate buffer and stir for 15 min;
5. Add Toynol® BWA-925 and stir for 15 min;
6. Add Vanconol® MGR-83 and stir for 30 min;
7. Add Toynol® DF-80 and stir for 15 min;
8. Add Toynol® Foamic-024 and stir for 10 min;
9. If used, add the silicate and stir for 10 min;
10. Add water to make up to the total volume and stir for 20 min;
11. Check the pH to 8.6–9.0 and fine-tune with triethanolamine;
12. Allow to stand and age for 24 h, check stability, and filter.

Process key points:

• Water quality first, buffering second;
• Add the wetting agent before the corrosion inhibitor to ensure uniform spreading;
• Staged defoaming: DF-80 molecular defoaming + Foamic-024 long-lasting foam suppression.

6. Dedicated Validation Plan for the White-Spot Problem

Validation tier	Validation content	Acceptance criterion
Basic validation	40℃, 3.5% NaCl, AZ31 coupon, 72 h	No white spots/pitting; corrosion inhibition rate ≥85%
Film integrity	Copper-accelerated acetic acid salt spray test (CASS), 48 h	White-spot area <5%
Flushing resistance	Simulated cutting circulation (flow rate 2-3 m/s) + coupon test	No film detachment, no localized white spots
Thermal stability	55℃ hot storage for 30 days + coupon test	Corrosion inhibition rate drop ≤10%
Actual machining validation	Machine AZ31 workpieces normally and leave for 48 h after cutting	No white spots or discoloration on the surface
Additive compatibility	MGR-83 + Toynol® additives, 50℃ hot storage for 14 days	No precipitation, no phase separation, stable pH

7. Selection Basis for Toynol® Additives

Product	Key technical parameters	Application notes
BWA-925	Low foam, dynamic surface tension as low as 43.8 mN/m	Validated in coating systems; requires laboratory validation in cutting fluids
FS-204E	Dynamic surface tension as low as 32.4 mN/m	Official-website TDS data; requires validation in cutting fluids
DF-80	Acetylenic diol gemini-type defoamer, does not oil out	Validated in waterborne systems; cutting-fluid compatibility to be tested
Foamic-024	Epoxy-system dedicated defoamer, strong bubble breaking + long-lasting foam suppression	Suitable for alkaline systems; requires validation in cutting fluids

Application note: The Toynol® additive recommendations in this solution are based on functional matching with existing product technical data, and the fundamental wetting and defoaming performance of the products has already been maturely validated. Their synergistic anti-white-spot effect with the corrosion inhibitor in this specific magnesium alloy semi-synthetic cutting-fluid system should be further confirmed through the laboratory trials in the Section 6 validation plan. The fundamental resolution of the white-spot problem depends on the performance of the MGR-83 corrosion inhibitor and overall formulation optimization, with Toynol® additives playing a supporting role.

8. Summary of the Comprehensive Solution

8.1 Summary of Core Adjustments

Optimization dimension	Specific measures	Effect on the white-spot problem
Corrosion inhibitor	Maintain MGR-83 at 0.5%–1.0%	Primary protection, pH matched, high-temperature stable
Water-quality control	Must use deionized/softened water	Removes the Cl⁻ aggressive factor
Buffering enhancement	Borate buffer at 0.3%–0.5%	Stabilizes localized pH and prevents heat-induced fluctuation
Toynol® wetting	BWA-925 at 0.05%–0.15%	Low-foam uniform spreading, ensuring film continuity
Toynol® defoaming	DF-80 + Foamic-024 staged defoaming	Prevents foam from compromising film integrity
Film enhancement	Optional silicate at 0.1%–0.2%	Fills micro-defects

8.2 Recommended Combinations

Option	Toynol® combination	Features	Validation priority
Recommended combination	BWA-925 + DF-80 + Foamic-024	Full coverage, low-foam priority	Validate wetting first → defoaming → corrosion inhibition
Simplified combination	FS-204E + Foamic-024	Two-in-one simplification, integrated wetting and foam control	Validate compatibility → overall performance

8.3 Implementation Roadmap

1. Sample acquisition: contact Tianjin Hi-Perferal to obtain MGR-83 and Toynol® samples;
2. Water preparation: ensure the supply of deionized/softened water;
3. Laboratory trial design: dosage screening design (water-quality comparison, buffer gradient, additive-combination comparison);
4. Dedicated validation: focus on validating the white-spot prevention effect;
5. Actual machining test: small-batch actual-machining validation;
6. Process consolidation: optimize process parameters and establish an SOP.

9. Final Recommendations

9.1 Core Action Items for the White-Spot Problem

Must do:

• Switch to formulation with deionized/softened water;
• Add a borate buffer at 0.3%–0.5%;
• Prepare according to the optimized process sequence.

Recommended:

• Toynol® BWA-925 wetting + DF-80/Foamic-024 defoaming combination;
• Optional silicate addition at 0.1%–0.2%;
• Strengthen chip removal and concentration monitoring.

Validation focus:

• Dedicated white-spot/pitting testing, 72 h NaCl coupon;
• Film integrity evaluation, CASS or SVET;
• Post-machining workpiece observation, 48 h standing.

9.2 Process and Maintenance Recommendations

Management point	Recommended measure
Additive dosage	Confirm the optimal dosage through dosage screening trials before scaling up
Process specification	Develop a detailed SOP and train operators
Water-quality monitoring	Periodically test the Cl⁻ concentration of the diluted fluid, targeting ≤50 mg/L
Fluid change management	Recommend a 1–3 month fluid-change interval and establish a periodic testing regime