An electroplater wrote to me once that he knew absolutely nothing about magnesium.
Frank AltmayerUnfortunately, he was put in charge of the production of a part made of cast magnesium, and when they salt-spray these parts, they quickly corrode.
He wondered, “Why is our magnesium casting so poor in corrosion resistance?”
I looked at his submitted magnesium parts and found them in a “normal condition” after salt spray. Bare magnesium does not hold up well in salt spray because the metal is highly active (more than aluminum) and is easily converted to corrosion products.
Here is some basic information on magnesium.
Image courtesy of trodesigns.com
All About Magnesium
While some magnesium is found in ingot form on the ocean floor, most magnesium is produced by extracting magnesium chloride from seawater. The magnesium chloride is reduced to magnesium by electrolysis. Magnesium is the world’s lightest structural metal. It is typically alloyed with aluminum, manganese, zinc, thorium, and/or zirconium for increased strength.
- Magnesium alloys can be anodized, conversion coated, painted, electroplated, or mechanically finished.
- Magnesium can be cast by sand, die, permanent mold, and precision investment methods. It is usually extruded or rolled into sheets, plates, and strips. It can also be formed by drawing, bending, or spinning and is forgeable by press and hammer equipment.
- Magnesium alloys and their corrosion products present no (or low) toxicity hazard.
- Magnesium is considered the easiest metal to machine, but it can catch fire if not carefully handled. Magnesium dust and small particles can catch fire and/or explode because the metal rapidly combines with oxygen when given an opportunity. Therefore, machining and grinding must be carefully conducted.
Other significant characteristics of magnesium include good stability in atmospheric exposure and resistance to attack by many chemicals, including alkalis, chromic and hydrofluoric acids, and organic chemicals such as hydrocarbons, most alcohols (except methanol), phenols, amines, esters, and most oils. Magnesium is rapidly attacked by methanol and cannot be used in applications where magnesium alloys are exposed to solutions containing even small amounts of methanol.
Magnesium (and alloys) are nonmagnetic, have relatively high thermal/ electrical conductivity, and have good absorption characteristics for vibration and shock.
Magnesium is a very active metal that corrodes rapidly in the presence of many corrosives, including saltwater and most galvanic cells. Bare magnesium is used in a corrosive environment and requires a protective coating. Protective coatings that are commonly applied include the following.
Chromate Conversion Coating
Numerous chromate conversion treatments are in the literature. ASTM D1732 and MIL-M-3171 cover this process. Chromate conversion coatings are typically used as either an undercoat to promote paint adhesion or as stand-alone films for corrosion protection in storage and mildly corrosive environments.
Anodizing
The mechanism of magnesium anodization is significantly different from that of aluminum in that it occurs in alkaline solutions and through a sparking process. At potentials over a given voltage (usually near 50V DC), sparks form on the surface of the magnesium anode. These sparks move over the surface, and a film is produced where they travel. The film results from a chemical reaction between the anodizing bath's magnesium alloy, oxygen, electrolytes, and other components.
Because of the temperatures reached in a spark, a significant number of excited species are available to contribute to the chemical and electrochemical reactions. The phases include liquids, dissolved species, gases from electrolysis and boiling, and the solid electrode. Physical processes, such as fusion, can also occur at this temperature. Therefore, The process is very complex, and the variables contributing to the film formation are somewhat difficult to isolate.
Most anodizing treatments provide a hard, corrosion-resistant coating on the magnesium. The one exception is “galvanic” anodizing, or Chemical Treatment No. 9. The other anodizing treatments include Chemical Treatment No. 17,* HAE, and Cr-22. Magnesium anodizing is covered under MIL-M-45202.
Electroplating
Successfully plating magnesium alloys is an even greater challenge than plating aluminum. Magnesium alloying constituents can form localized cathodic/anodic sites that may have low hydrogen over-voltage potential, resulting in hydrogen evolution during plating processing. Magnesium parts often exhibit surface contamination from manufacturing processes, such as polishing and buffing. Differences in surface composition and potential can result in skip plating if pre-plate procedures are inadequate.
The processing steps for plating magnesium parts are similar to those for processing aluminum, but the solutions are significantly different. Magnesium has a thin skin of oxides containing graphite and other localized inclusions. The acid pickle (30-60 sec at room temperature) and activation step (30-120 sec at room temperature) must remove this skin for successful plating.
Following cleaning, parts may be pickled in chromic or phosphoric acid pickles. An activation step, consisting of 20 vol% phosphoric acid and 100 g/L of sodium acid fluoride (NaHF2), follows the acid pickle and precedes the immersion zinc. The immersion zinc is operated at 155-160°F. The pyrophosphate assists in the formation of an adherent zinc deposit on magnesium by reacting with any surface oxides, dissolving and complexing them. Fluoride is present to control the deposition rate. Zincate immersion time is typically two minutes but may take up to 15 minutes. No benefit from “double zincating” magnesium alloys has been reported.
A stainless steel tank and heating coil are typically used. Close control of the narrow pH window is very important.
The initial cyanide copper strike is operated at a low current density (5-10A/ft2). No gassing should occur, or blistering may result. After two minutes, the current may be doubled. Periodic reverse (30 sec forward, followed by 15 sec forward, 3 sec reverse) has been proven to provide superior results.
Magnesium Uses
Magnesium is used in aircraft parts, textile machinery parts, printing machinery parts, hand trucks, grain shovels, gravity conveyors, foundry equipment, and other products in the materials-handling field that require a low weight-to-strength ratio. The portable tool industry utilizes magnesium castings for chain saws, drills, impact hammers, grease guns, and other manually handled articles. Other common applications include household goods, office equipment, cameras, golf clubs, luggage, and vehicles.
Magnesium Nomenclature
The standard nomenclature for magnesium alloys was adopted in 1948 and is explained as follows:
- First two characters: Identify the main alloying elements. Example: AZ= Aluminum & Zinc. Other letters: B=Bismuth, C=Copper, D=Cadmium, E=Rare earth, F=Iron, H=Thorium, K=Zirconium, L-Beryllium, M= Manganese, N=Nickel, P=Lead, Q=Silver, R-Chromium, S=Silicon, T=Tin.
- Next two characters: Indicate the percent of the main alloying elements. For example, AZ31 = 3% Al, 1% Zr.
- Next character: Separates different alloys containing the same percent primary elements. For example, AZ91A and AZ91B differ in copper content but have the same amount of primary alloying elements.
- Dash and next two characters: Indicates surface condition, similar to aluminum nomenclature. Example: AZ31A-T6 = Solution heat treated and artificially aged.
Frank Altmayer is a Master Surface Finisher, an AESF Fellow, and the technical education director of the AESF Foundation and NASF. He owned Scientific Control Laboratories from 1986 to 2007 and has over 50 years of experience in metal finishing. He received the AESF Past Presidents Award, NAMF Award of Special Recognition, AESF Leadership Award, AESF Fellowship Award, Chicago Branch AESF Geldzahler Service Award, and NASF Award of Special Recognition.
