Detailed explanation: golden yellow passivation (imitation gold) process and troubleshooting of zinc coating

· 1 Preface ·

Imitation gold plating, usually first plating bright nickel layer or “bright copper-bright nickel” as the bottom layer, and then plating 1-2μm thinner copper alloy, such as cyanide plating Copper zinc, copper-tin or copper-zinc-tin alloy method to obtain a decorative imitation gold plating layer.

In recent years, the galvanized layer dyeing and varnish imitation gold process introduced in recent years is to passivate the galvanized layer with a golden passivation solution to form a golden passivation After the film is washed, it is immersed in a dehydrating anti-rust agent to obtain a uniform golden color with a fixed film layer, and then coated with a transparent organic film (varnish) with a certain hardness for protection, so as to realize the production of imitation gold decoration.

· 2 Process flow ·

Iron and steel products→pretreatment→galvanizing→flowing water cleaning→lye rinsing→second flow water cleaning→zinc layer light emission→second flow water cleaning→golden passivation→flow water cleaning→dipping dehydration antirust agent →Running water cleaning→Film aging→coating clear paint→organic film curing→operator self-inspection→full-time inspection of finished product packaging and storage.

· 3 Process formula ·

◆◆ Light emitting liquid: nitric acid 3%-5%, table salt 5%-10%.

◆◆ Passivation solution: chromic acid 3-5g/l, sulfuric acid 0.3-0.5g/l, nitric acid 0.7-0.9g/l, room temperature, time 10-30s, idle 8-10s.

◆◆ Anti-rust agent: polyvinyl alcohol 15-25g/l, sodium benzoate 30-50g/l, sodium nitrite 20-30g/l, triethanolamine 50-100g/l, ethanol (1:2) 800ml /l. Preparation instructions: Pour a calculated amount of ethanol into the enamel barrel and heat it to 90-95°C, slowly add polyvinyl alcohol under constant stirring to dissolve it into a colloidal liquid. Cool to 40-50°C, add the calculated sodium benzoate, sodium nitrite and triethanolamine in sequence, stir and dissolve to form a transparent liquid, or use a commercially available dehydrating rust inhibitor suitable for this process.

◆◆ Coating: 604 epoxy resin 50%, propionyl 20%, butanol 10%, xylene 10% cyclohexanone 10% or use commercially available coatings, such as acrylic varnish, modified acrylates Paint, etc.

· 4 Notes ·

(1) To obtain the ideal golden yellow quality, a good galvanized layer is an important part. All galvanized coatings such as cyanide, alkaline and acidic zinc coatings can be produced for imitation gold decoration.

However, the galvanized layer must be smooth, compact, and bright, and the coating must contain less inorganic and organic impurities. Therefore, it is necessary to filter the plating solution frequently to minimize the accumulation of organic impurities and metal impurities, because the accumulation of impurities in the bath solution and the amount of impurities contained in the galvanized layer also make the golden passivation film more likely to change color.

(2) Galvanized gold imitation products generally require rack plating. If they are barrel-plated parts, they should be immersed in clean water or dilute lye after exiting the tank, and then the plating parts should be installed on the rack for passivation. Treatment to ensure that the zinc layer is fully passivated on the golden yellow film, otherwise the film will be uneven or partially dropped due to overlap or collision of the parts.

(3) Before the parts are galvanized, they must be mechanically polished and then degreasing treatment. No matter what degreasing method is used, the grease and dirt on the surface of the parts must be thoroughly removed to ensure good adhesion of the galvanized layer.

(4) Since the decomposition products of the zinc plating brightener are easily adsorbed on the surface of the zinc layer, the washing performance is not good, so the parts need to be rinsed with dilute sodium hydroxide (about 3g/L) lye after cleaning to make organic matter It is removed from the surface of the zinc layer to provide a strong guarantee for the strong bond of the golden passivation film.

(5) After the galvanized layer is exposed to light, it must be fully cleaned to prevent impurities from being carried into the passivation tank, which will affect the passivation effect, and immediately move it into the passivation solution for passivation. Shake gently, don’t collide with each other, to prevent the film from falling off. When exiting the tank, be sure to stay in the air for 8-10s and then clean it to improve the quality of film formation.

(6) Pay attention to the changes in the thickness and color of the passivation film. This must strictly control the passivation time. This process specification is 10-30s, such as the barrel galvanizing layer, about 10s. If the passivation time is too short, the film will be too thin, and if the time is too long, the film will be dissolved. The length of the passivation time should be flexibly controlled according to the use of the passivation solution, such as the new night time should be shorter, otherwise it can be longer.

(7) After the parts are golden-yellow passivated, they must be immersed in a dehydrating rust inhibitor immediately after cleaning for a while, so as not to lose water and cause the film to discolor or form rainbow colors. Because only in the case of immersion in the anti-rust agent, a transparent film sealing layer can be effectively formed, and the moisture on the surface of the passivation film can be removed, so as to obtain a uniform golden yellow.

(8) After the parts are immersed in the dehydrating anti-rust agent, the temperature should not be too high when they are put into the oven for aging. Generally, the temperature should be controlled at 40-50℃, and the time should be about 15min. Then a layer of transparent organic film is coated and cured for protection.

·5 Troubleshooting·

During the golden passivation process of the galvanized layer, the common failure phenomena, main reasons and troubleshooting methods (see the table below).

Common failure causes and troubleshooting