How to Select Corrosion Inhibitors for Magnesium Alloys? Prevention and Control of White Spots in AZ31 Machining
1. First Diagnose: What Do White Spots Look Like and What Corrosion Do They Correspond To?
AZ31 is one of the most widely used wrought magnesium alloys (approximately 3 % aluminum, 1 % zinc). With low density and high specific strength, it is widely used in 3C products, automotive lightweighting, and aerospace. However, magnesium is one of the most reactive common metals (standard electrode potential approximately -2.37 V) and is highly susceptible to anodic dissolution in aqueous media. White spots appearing after machining are the “on-site evidence” of corrosion.
White spots are not a single morphology. Distinguish the type before treating:
| White spot morphology | Typical cause | Investigation direction |
|---|---|---|
| Uniform white powder coverage | Uniform corrosion: overall attack by the machining fluid; pH out of control or elevated chloride ion content | Measure machining fluid pH and chloride ion content |
| Pitting white spots / corrosion pits | Pitting corrosion: chloride ions break down the surface film, locally accelerating dissolution | Identify chloride sources (chlorine-containing additives, tap-water make-up) |
| White spots on machined surfaces / edges | Micro-galvanic corrosion: potential difference between second phases and the matrix, with machined stress zones being more reactive | Check alloy batch, machining heat, and cooling conditions |
| White-gray ash during storage | Missing inter-process rust prevention: incomplete cleaning or wet parts stacked together | Check cleaning and drying processes, and rust-preventive fluid usage |
In brief: White spots = corroded magnesium matrix + accumulation of corrosion products. First identify the corrosion type from the morphology, then treat accordingly—this is more effective than adding chemicals blindly.
2. Machining Fluid Management: The Primary External Cause of White Spots
Corrosion inhibitors are not a catch-all—when the machining fluid environment is out of control, even the best inhibitor performs far worse. Three baseline requirements for magnesium alloy machining fluid management:
- Low chloride: avoid chlorine-containing additives, use deionized water for make-up, and periodically test for chloride ion enrichment (aging machining fluid concentrates chloride ions);
- Alkaline and stable pH: magnesium corrodes at a lower rate in the alkaline range (approximately pH 8.5–11); pH fluctuation or acidification destroys the surface film;
- Set baselines by fluid type: magnesium alloys are demanding on machining fluids; different types have different compatibility:
| Machining fluid type | Characteristics | Magnesium alloy machining considerations |
|---|---|---|
| Emulsion | Good lubrication, but high oil content, prone to bacterial growth, pH drift | Strictly monitor pH and bacterial count; chloride ions tend to concentrate |
| Semi-synthetic fluid | Balanced lubrication and cleaning; a common direction for magnesium alloy machining | Vanconol® MGR-83 is the optimized solution for AZ31 semi-synthetic cutting fluids (solution article on the official website) |
| Fully synthetic fluid | Good cleaning and cooling, easy to maintain, but weaker lubrication | Corrosion inhibitor needed to supplement corrosion protection; check compatibility of extreme-pressure additives with magnesium |
In brief: Get the machining fluid right first (low chloride, stable alkaline pH, baselines by fluid type) so that the inhibitor has room to perform—this is the foundation of white spot prevention.
3. Vanconol® Corrosion Inhibitor Solutions: Matching Product Grades by Operating Conditions
Vanconol® (维克乐®) is positioned as a corrosion protection specialist. The primary corrosion inhibitor components for magnesium alloys are independently developed; the grades sold on the official website are matched to operating conditions:
| Operating condition | Vanconol® recommendation | Data to verify on official website |
|---|---|---|
| General magnesium alloy protection (automotive / 3C / aerospace) | MGR-82 | Inorganic-organic composite chromium-free magnesium alloy corrosion inhibitor; 91 % corrosion inhibition rate after 24 h immersion in 3.5 % saline solution (official website data) |
| AZ31 semi-synthetic cutting fluid | MGR-83 | Chromium-free waterborne low-VOC magnesium alloy corrosion inhibitor; primary inhibitor in the optimized AZ31 semi-synthetic cutting fluid solution (official website solution article); resistant to damp heat, salt spray, and acidic environments (official website English description) |
| Aluminum-magnesium alloy systems (core of aluminum alloy machining fluids) | ALI-910 | C9–10 polyether phosphate (anionic); withstands 200 g/L NaOH and ≤90 °C; core of aluminum alloy machining fluids (official website data) |
Selection in three steps:
- Determine fluid type: start with MGR-83 for semi-synthetic cutting fluids; evaluate the compatibility of MGR-82 with emulsion / fully synthetic systems;
- Determine dosage: set up 3–5 dosage levels from the TDS-recommended starting dose; observe white spot inhibition and machining fluid stability;
- Determine combination: for mixed aluminum-magnesium processing lines, evaluate combinations of ALI-910 with the MGR series; match by system and do not mix aluminum-specific and magnesium-specific corrosion inhibitors.
Data note: public reports indicate that MGR-83 at 0.6 % addition in a 50 % ethylene glycol system provides 7-day protection for AZ91D magnesium alloy (slight surface dulling with good protection retained). This data is from public reports; for formal use, follow official TDS actual test results.
In brief: Magnesium alloy corrosion inhibitor selection = first determine the machining fluid type → match by operating conditions (MGR-83 for AZ31 semi-synthetic fluids, MGR-82 for general magnesium alloy protection, ALI-910 for aluminum-magnesium systems) → use gradient small-scale testing to set the dosage.
4. Implementation Path and Common Pitfalls
Closed-loop implementation (four steps):
- Baseline testing: machining fluid pH, chloride content, and concentration must be up to standard before considering the inhibitor;
- Gradient small-scale testing: set 3–5 dosage levels from the TDS starting dose; immerse for 24 h / 72 h; observe white spot formation and weight loss; calculate the corrosion inhibition rate;
- Operating condition validation: small-batch trial machining plus stacked-plate storage tests; verify inter-process rust prevention and the interface with subsequent surface treatment (anodizing / coating);
- Continuous monitoring: periodically test chloride content and pH of the machining fluid; replace at scheduled intervals; record data for closed-loop management.
Common pitfalls:
- White spots ≠ oil contamination: white spots are corrosion products and cannot be washed off with water. Weak acid or specialized magnesium alloy cleaners can remove them, but improper pickling exacerbates corrosion—test on a small area first;
- Inhibitors do not replace machining fluid management: when chloride is too high or pH is out of control, inhibitor performance drops significantly;
- Aluminum inhibitors ≠ magnesium inhibitors: magnesium alloy corrosion inhibitors are designed for magnesium systems (MGR series); distinguish clearly during selection and do not mix;
- Chromium-free ≠ harmless: MGR-82/83 are chromium-free and more environmentally friendly, but still require handling per the SDS; dispose of waste fluid per local environmental requirements;
- Pretreatment interface: for downstream anodizing or chemical polishing, residual inhibitor may affect the film layer—evaluate the cleaning process interface.
FAQ
Q1: What causes white spots after AZ31 magnesium alloy machining?
A: The magnesium matrix is corroded in the machining fluid (micro-galvanic corrosion plus chloride ion attack); corrosion products such as magnesium hydroxide / basic magnesium carbonate accumulate as white powdery spots. High chloride content and improper pH in the machining fluid aggravate the problem.
Q2: How can white spots be removed from magnesium alloys?
A: White spots cannot be washed off with water (they are corrosion products, not oil contamination). Use a specialized magnesium alloy cleaner or weak acid cleaning. Improper pickling worsens corrosion; recommended practice is to test on a small area to confirm the process before batch treatment.
Q3: Which corrosion inhibitor should be used for AZ31 semi-synthetic cutting fluid?
A: Vanconol® MGR-83 can be evaluated (chromium-free waterborne low-VOC; primary inhibitor in the optimized AZ31 semi-synthetic cutting fluid solution). In parallel, ensure the machining fluid is low-chloride with a stable alkaline pH. Dosage is determined per TDS and gradient small-scale testing.
Q4: Can magnesium alloy and aluminum alloy corrosion inhibitors be used interchangeably?
A: Not recommended. Magnesium alloy inhibitors (e.g., MGR-82/83) are designed for magnesium systems; aluminum inhibitors are not necessarily suitable for magnesium alloys. For mixed aluminum-magnesium lines, evaluate combinations of ALI-910 with the MGR series and match by system.
Q5: Is the corrosion inhibition rate data for MGR-82 reliable?
A: The 91 % corrosion inhibition rate after 24 h immersion in 3.5 % saline is data published on the official website. For formal use, please verify against Hi-Perferal’s official TDS and small-scale testing under your actual operating conditions.
Core Conclusions
- AZ31 white spots = magnesium matrix corrosion + product accumulation: first diagnose the corrosion type by morphology (uniform / pitting / micro-galvanic), then treat accordingly
- Three-layer prevention: low-chloride alkaline machining fluid management + magnesium-specific corrosion inhibitors + inter-process cleaning and drying—all are indispensable
- Inhibitors matched by fluid type: MGR-83 for AZ31 semi-synthetic cutting fluids; MGR-82 for general magnesium alloy protection (91 % corrosion inhibition rate in 3.5 % saline for 24 h, per official website); ALI-910 for aluminum-magnesium systems
- Inhibitor effectiveness is subject to small-scale validation under actual operating conditions; data is based on TDS and actual measurements
