Organic coating treatment of magnesium alloy

In order to improve the corrosion resistance of magnesium alloy workpiece and its products, moisture resistance, heat resistance, thermal insulation performance and decorative effect, can be coated on it in a variety of paint, oil and grease protection effect is not long, however, a lot of troubles and associated, so organic coating is widely used, the commonly used organic coatings with paraffin wax, bitumen, rubber, plastic, organic polymer, etc.The research and practice show that the epoxy resin has a strong adhesion with magnesium and magnesium alloy, does not wet with water, nor does any reaction, and has a very high strength, is a good coating, in addition, ethylene resin and polyurethane also obtained more applications.Studies have pointed out that the magnesium alloy containing PPY coating in the salt spray chamber even if placed for 1000h, there is only a trace of corrosion and a little foaming, showing good corrosion resistance.

If the workpiece is transformed or anodized before the organic coating is applied, the corrosion resistance life of the coating can be further extended. Pretreatment can roughen the surface of the magnesium workpiece and improve its chemical properties, which can significantly improve the adhesion between the magnesium surface and the coating.However, not every coating is pretreated before it is applied.Some automotive die castings do not require pretreatment.After pretreatment, the workpiece must be well dried to remove all surface moisture.Studies have shown that the corrosion resistance of anodized workpieces can be greatly improved if the pores in the workpieces are sealed with a permeable resin prior to the application of a primer or top coat. Anodized coatings that are not sealed with paint or resin should not be used in saline environments.

Organic coatings (primers or single-layer coatings) in direct contact with magnesium surfaces must have good alkalinity resistance. The best organic coatings are ethylene or epoxy polymers.Roasting maximizes the removal of solvents and the cross-linking of polymers.The baking temperature depends on the effect on the properties of the alloy and the thermal stability of the conversion coating used.The aging temperature of AZ31B alloy sheet is less than 150℃;The baking temperature of chromate treated die casting can reach 200 ° C, and the silicon surface coating of some aerospace magnesium alloys that have been anodized can withstand 315 ° C.

Epoxy surface seal of magnesium alloy

Resin formula (quality) : epoxy resin HZ985 hardener 100 parts, epoxy resin PZ985 300 parts, ethyl acetate 240 parts, toluene 136 parts, diacetone alcohol 24 parts.

The above formula solution is suitable for dip coating, if add solvent after dilution can also spray;Usually 25% will do, but for surfaces with strict deviations, more solvent should be added.

Coating process.Whether spraying or dipping epoxy resin, the process is: preheating at (200 ~ 220) ℃/30min, or after reaching the preheating temperature, holding for 10min, cooling to 60℃, coated with Araldite PZ/HZ285 resin;After drying in the air for 15min ~ 30min, remove lacrimal stripe with a small brush to ensure uniform coating, bake at 200℃ ~ 220℃ for 10min ~ 15min;Remove liquid droplets or teardrops that may be on the coating, and work with care not to damage chromate or anodized film.Repeat the above operation 2 or more times, that is, a total of 3 or 4 coats.The final process is baking at 200℃ ~ 220℃ for 45min.The coating thickness thus formed is about 0.025mm.

primer

Magnesium alloy primers must be alkali resistant, such as primers based on polyvinyl butyral, acrylic acid, polyurethane, baking phenolic resin, and alkene-based epoxy.For improved corrosion resistance, zinc chromate or titanium dioxide can be added to the primer.Most primers contain epoxy resins, which provide good alkalinity resistance.

To prevent foaming, a vinyl-etch wash with very low phosphoric acid content, such as less than 75%, can be used, but it is often followed by an epoxy primer. For parts that work in harsh environments such as the ocean, strontium chromate or other chromates should be added to the primer.However, under condensation humidity or water immersion conditions, paint containing chromic acid or other slightly soluble salts can promote foaming.Magnesium primers should not contain heavy metals such as lead or copper compounds.In addition, coatings containing zinc that have been successfully applied to steel shall not be applied to magnesium and magnesium alloys.

Called E - of cathodic electrodeposition coating epoxy primer in the industry won a widely used effective, particularly for handling car and computer die casting, it besides has the good adhesion, and excellent properties, both in the complex surface (including the deep cavity) formed on the uniform coating, the coating process of the final thickness from 0.018 mm to 0.025 mm, usually it is used as the ground floor, but in special circumstances can also be used as a protective coating.

paint

Magnesium alloy workpiece finish requirements: good compatibility and protective performance with primer;Long - term stable work in the working environment, including outdoor exposure to ultraviolet radiation.In addition, the choice of finish coat should also consider the economical reasonableness of meeting the application requirements, the conditions of use and the compatibility with existing coating equipment.

Topcoats that have been successfully used in the magnesium industry are alkyd resins, epoxy resins, polyesters, propionic acids and polyurethane.Self-drying varnish and baking drying varnish, the latter is harder and can better resist solvent erosion.Vinyl acid resins have alkali resistance, acrylic resins have strong salt resistance, alkyd resins can be used outdoors for a long time, and epoxy resins have good wear resistance, vinyl resins can be used above 150℃.Other surfaces have higher heat resistance and the heat resistance temperature increases in the following order: modified vinyl resin, epoxy resin, modified epoxy resin, epoxy-siloxane, siloxane resin.

Electrostatic spraying method and fluidized bed method have been widely used to protect the workpiece of magnesium alloy. In addition to good film forming performance, powder coating is also beneficial to environmental protection.Epoxy, polyester and their mixtures are effective topcoats for magnesium alloy luggage racks, arrow bows, computer cases, tool boxes and other automotive parts.

The workpiece must be cleaned and surface prepared before powder coating. The steps and requirements are the same as those for liquid coating of magnesium parts. Depending on the application requirements, the process is the same as that for phosphorus salt conversion coating.

The "degassing bubble" defect may occur in the magnesium alloy workpiece during powder coating, but it is less and smaller than that of liquid coating.The following measures can be taken to control: strictly abide by the casting process, reduce the surface pores;The casting is pre-baked to a temperature slightly higher than the curing process;Coating with low curing temperature;Choose a paint with a long "open" time that allows generated gases to be expelled before the coating fully solidifies.