Magnesium Chemical Conversion Coating
Anodizing & conversion coatings · Chemical bath
Magnesium conversion coating reacts with magnesium alloys to improve corrosion resistance and paint adhesion with minimal buildup.
Reference page — Mag conversion is not in the instant-quote configurator. Upload the part, name the finish in the notes, and an engineer quotes it with the part.
What is Mag conversion?
Legacy Dow processes and modern chrome-free systems are selected under AMS-M-3171 or SAE AMS 3171 and processor-specific qualification.
How it works
After alkaline cleaning and acid activation, bare magnesium is immersed in or sprayed with a reactive conversion solution. Legacy Dow 7-type chromate chemistry forms a protective chromium-bearing film; modern qualified systems use phosphate, permanganate, zirconium, or other chrome-free chemistry. The film is thin, generally about 0.5–5 µm, and is usually painted or sealed because magnesium is highly galvanically active. HAE is often grouped commercially with magnesium treatments but uses electrolytic anodic oxidation and should not be assumed equivalent to chemical conversion.
Why specify it
- Thin 0.5–5 µm film limits dimensional change
- Improves primer and paint adhesion
- Provides temporary corrosion protection to reactive magnesium
- Chrome-free qualified options avoid legacy hexavalent chromium
Where it falls short
- Bare conversion film is rarely enough for severe service
- Legacy Dow 7 chemistry contains regulated chromium
- Alloy and casting porosity affect uniformity
- HAE anodizing is not chemically equivalent to Dow 7 conversion
Mag conversion at a glance
Mag conversion is a middle-of-the-range finish; its best score is corrosion protection (42 of 100), at a premium cost. It adds little lubricity. Surface hardness is not rated: the finish leaves the part's own surface.
Each score is 0–100 against every finish in this catalog. Corrosion comes from salt-spray life (ASTM B117, log scale) and hardness from the finish layer's Vickers or pencil hardness; cosmetic, lubricity and cost are FabDigit's engineering assessment. Economy on the radar is the inverse of cost.
Mag conversion key numbers
Typical values from the governing specifications and shop practice; bars are this finish against the highest value among anodizing & conversion coatings finishes.
- Thickness
- Approximately 0.5–5 µm for chemical conversion films; electrolytic HAE treatments can differ.
- Electrical
- Semi-conductive
- Colours
- clear
- gold
- brown
- gray
- Appearance
- Clear, gold, brown, or gray depending on legacy chromate or chrome-free chemistry.
Specifications for Mag conversion
The documents a drawing cites to define this finish, with the Type / Class rows that apply. Each links to a page explaining the standard.
| Standard | Type / Class | Thickness | Requirement / test | Status |
|---|---|---|---|---|
Magnesium Alloy, Processes for Pretreatment and Prevention of Corrosion on · SAE | Type I | — | Legacy Dow 1 chemical pickle for temporary protection and paint preparation. · Appearance and process control. | Active |
| Type III | — | Legacy Dow 7 dichromate treatment for magnesium alloys. · Appearance and process control. | ||
| Type IV | — | Legacy Dow 9 galvanic anodic process. · Appearance and process control. | ||
| Type VI | — | Legacy Dow 19 treatment used for local application and touch-up. · Appearance and process control. | ||
| Type VII | — | Electrolytic fluoride-based treatment followed by specified post-treatment. · Process qualification and appearance. | ||
| Type VIII | — | Chromate treatment used as protective coating and paint base. · Appearance and process control. | ||
Magnesium Alloy, Processes for Pretreatment and Prevention of Corrosion on · SAE · alternative | Types I–VIII | — | Chemical and anodic processes selected by exact type; they are not performance-equivalent. · Tests and process controls applicable to the selected type. | Active |
Test and related methods: ASTM B117 (Standard Practice for Operating Salt Spray (Fog) Apparatus).
Drawing call-out
CHEMICAL TREAT MAGNESIUM PER SAE AMS-M-3171 / AMS 3171, CHROME-FREE CONVERSION, PRIME WITHIN 8 HOURS; DO NOT SUBSTITUTE HAE ANODIZEPattern: TREAT MAGNESIUM PER AMS-M-3171, TYPE <I|III|IV|VI|VII|VIII>, <PAINT BASE|TEMPORARY PROTECTION>, <MASKING>
Which materials take Mag conversion?
Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library.
| Material family | Verdict | Notes |
|---|---|---|
| Yes | The intended substrate; qualification must match alloy and casting condition.
| |
| Limited | Only if the AM part is a magnesium alloy and the chemistry is qualified for that powder. | |
| No | Use an aluminum-specific conversion coating. | |
| No | Brass is not a magnesium-conversion substrate. | |
| No | Copper alloys are not magnesium-conversion substrates. | |
| No | Fused-filament polymers are incompatible with this process. | |
| No | Polymer powder-bed prints are incompatible with this process. | |
| No | Organic polymers cannot take this inorganic metal-finishing process. | |
| No | Photopolymer prints are incompatible with this process. | |
| No | Polymer powder-bed prints are incompatible with this process. | |
| No | Stainless steel is not treated by magnesium conversion baths. | |
| No | This conversion chemistry is for magnesium, not steel. | |
| No | Titanium uses a different treatment. | |
| No | Zinc alloys use different conversion chemistry. |
How Mag conversion is applied, and what it does to the part
- 1Clean
- 2Acid activate
- 3Rinse
- 4Apply qualified conversion treatment
- 5Dry
- 6Prime / seal promptly
| Dimensions | Chemical conversion buildup is usually only 0.5–5 µm, but HAE anodic treatments require separate allowance data. |
|---|---|
| Build-up per side | 0.5–5 µm — Chemical conversion has minimal buildup; do not apply this range to HAE anodic coatings without supplier data. |
| Masking | Usually needed. Mask electrical contacts when contact resistance is critical and isolate all dissimilar-metal inserts from process chemistry. |
| Electrical | Semi-conductive |
| Appearance | Clear, gold, brown, or gray depending on legacy chromate or chrome-free chemistry. |
| Added lead time | 4–8 working days on top of the bare part |
Design rules for Mag conversion
What to decide on the drawing before the part goes to the finisher.
- Name the exact AMS type and approved chemistry.Dow, HAE, and chrome-free systems are not interchangeable.
- Prime or seal promptly.Bare magnesium corrodes rapidly after activation.
- Seal casting porosity before cosmetic acceptance.Entrapped solution can bleed and stain.
- Design galvanic isolation from dissimilar fasteners.Conversion coating alone cannot stop a strong galvanic couple.
Mag conversion versus the alternatives
Finishes engineers weigh against Mag conversion, and when each wins.
| Finish | Corrosion | Hardness | Cosmetic | Cost | Thickness | Choose it when |
|---|---|---|---|---|---|---|
Mag conversion this page | 42 | — | 25 | $$$ | 0.5–5 µm | — |
| 70 | 74 | 35 | $$$$ | 10–150 µm | when a thicker ceramic wear and dielectric layer is required |
The radar overlays Mag conversion with Plasma Electrolytic Oxidation, its most common alternative — chosen when a thicker ceramic wear and dielectric layer is required. Scores in the table are 0–100 across every finish in the catalog.
Cost, lead time and compliance
Relative cost
Magnesium Chemical Conversion Coating is a premium-cost finish ($$$): cheaper than about 52 % of the finishes FabDigit quotes.
Finishing is priced per part with a lot minimum, from FabDigit's own China-side rates. The exact figure appears on the instant quote once the part is uploaded.
Lead time and compliance
| Added lead time | 4–8 working days |
|---|---|
| RoHS | Conditional |
| Notes | Legacy chromate systems may contain hexavalent chromium; qualified chrome-free systems can meet current restrictions. |
What Mag conversion is used for
- Magnesium aerospace castings
- Electronic housings
- Lightweight gear housings
- Painted magnesium brackets
Mag conversion — frequently asked
What conversion coating is used on magnesium?
Legacy chromate systems such as Dow 7 and qualified chrome-free systems are used under specifications such as AMS-M-3171 or AMS 3171. The exact type and approved chemistry must be stated.
How thick is magnesium conversion coating?
Chemical conversion films are typically about 0.5–5 µm. HAE is an electrolytic anodic treatment and can require different thickness data.
Is Dow 7 RoHS compliant?
Legacy Dow 7-type chromate chemistry generally requires hexavalent-chromium review and should not be assumed compliant. Specify a qualified chrome-free AMS process when regulation requires it.
Is HAE a magnesium conversion coating?
HAE is commonly discussed with magnesium surface treatments, but it is generally an electrolytic anodizing process. It should not be treated as chemically identical to an immersion conversion film.
Does magnesium conversion coating prevent corrosion?
It improves short-term protection and paint adhesion, but severe service usually requires primer and topcoat. Galvanic isolation remains essential because magnesium is highly active.
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