November 2016 Archives

Because of the aesthetic appearance and anticorrosion performance along with superior resistance to sulfur and heat in particular, molten aluminum plating is a popular technique applied in both batch and continuous production systems.

(1)Plating

(1)Batch production

Batch plating will be applied for single item groups, such as tubing, rolled steel, cast and wrought products. The plating processes are shown in the below figure.

Since the plating bath temperature becomes as high as 700℃, the flux has been specially developed for this temperature. The flux is made primarily of chloride compounds, such as potassium chloride, and a mixture of fluoride compounds, aluminum fluoride for example. The figure shown here is a process of dry flux treatment. In addition, a wet-type system where the flux is melted on top of the plating bath is also available. AL-Si (Si 7.5 to 9.0%) alloy seems to be the common composition of the plating bath.

(2)Continuous production

The following figure is an example of the continuous production line where plating is applied on strip steel plates.

At 500℃, grease and organic substances on steel plate surfaces will be oxidized and combusted. In the next step, the steel will be annealed in the annealing furnace (850 to 900℃) filled with ammonia decomposition gas where a reduction occurs. In the cooling zone, the steel plates will be cooled down to the temperature of the plating. Without being exposed to air, the steel plates are plated in the aluminum-plating bath.
The snout in the final cooling zone of this figure is used for inputting alkali metals such as sodium, which will be evaporated to generate sodium aluminate on the plating bath surface. This substance inhibits the generation of aluminum oxides or nitrides and keeps the plating surface smooth.

Problems related to hexavalent chromium have been attracting public attention recently. The problems are divided into two categories. The first one is hexavalent chromium generated from plating and chemical conversion processes. The other is hexavalent chromium remaining in chemical conversion coatings. In this volume, we will look into the first category.

(1) Constant effluent

(1) Reduction method

As in the case with cyanogen, it is necessary to detoxify a constant effluent and concentrated effluent solution containing hexavalent chromium. A constant effluent discharged from the chromate processing after chrome plating or zinc plating is generally treated by the reduction or ion-exchange method as described here.

1. Acidize effluent by adding sulfuric acid (pH2 to 3), and then add reducing agents such as sodium bisulfite to reduce hexavalent chromium (chromic acid) to trivalent chromium (chromium sulfate).
   4H₂CrO₄ + 6NaHSO₃ + 3H₂SO₄ → 2Cr₂(SO₄) ₃ + 3Na₂SO₄ + 10H₂O
2. When chromium sulfate, which is trivalent chromium, is neutralized (pH9) by adding alkali, it is precipitated as chromium hydroxide and you can separate it.
   Cr₂(SO₄) ₃ + 6NaOH → 2Cr(OH) ₃ + 3Na₂SO₄

(2) Ion-exchange method

This method adsorbs and removes anion (= chromic acid) using anion exchange resin. [Fig.1] is an example of the flow diagram.
The ion exchange resin used to adsorb chromic acid will eventually become saturated and no longer able to adsorb more ions. It is important to replace the resin before hexavalent chromium starts leaking.
The saturated ion-exchange resin will be collected by a chromic acid manufacturer, who regenerates the resin and recycles the eluted chromic acid. The regenerated ion-exchange resin is returned to the surface treatment plant.

(2) Concentrated effluent containing chromic acid

It is difficult for the plants to perform in-plant treatment of concentrated effluent containing chromic acid, such as waste chromium plating solution and waste chromate solution. Therefore, waste management system has been established where the chromic acid manufacturers collect such effluent and detoxify or recycle it.

Cyanide and hexavalent chromium are specified as harmful substances. From this volume, we will see how they are actually detoxified. Let's start with cyanide detoxification.

(1)Treatment of constant effluent containing cyanide

A method called alkaline chlorination is adopted for detoxification. This process alkalifies cyanide effluent and dissolves it into carbon dioxide and nitrogen by the action of chlorine. This process is usually performed in the following three stages:

(2)Highly concentrated effluent

For highly concentrated cyanide effluent, including old plating solution, plating film detachment solution, and exhaust gas cleaning solution, disposal is handled by an industrial waste treatment facility, instead of each plant detoxifying the effluent. Such companies are required to fully comply with the specified procedures after detoxifying the industrial waste by designated methods, such as the high-pressure hydrolysis method and electrolytic oxidation method.

(3)Incidents

From time to time, there are incidents where the cyanogen level exceeds the effluent standards. Most of such incidents are caused by incomplete separation of drainage, resulting in such effluent mixed into unintended drainage, not because of the inadequate cyanogen treatment.

This section of JIS defines silver coatings with the thickness of at least 0.5 µm on a metal or nonmetal substrate, produced for engineering purposes, including electrical/electronic products, machinery and other functional components.

(1) Appearance

The significant surfaces of the plating should be free from defects of polish, radiance, color shades, and imperfections as roughness, burnt parts, pits, dendrites on the edges, plating defects as substrate or undercoat exposure, adhesion failure as swelling or external flaws including smudges and scratches.

(2) Plating thickness

[Table] shows the minimum plating thickness of significant surfaces.

[Table] Classification by the minimum plating thickness

(3) Silver content

If the silver content in plating is specified, perform a test agreed between the parties concerned and display the value including the first decimal place.

(4) Porosity of plating

If the porosity of plating is specified, expose the sample into nitric acid vapor and check the number of corrosion points. Place a degreased and dried sample in the desiccator containing nitric acid. Leave it in there for one hour and measure the number of corrosion points per 1cm² as the porosity.

(5) Color fastness of plating

For anti-tarnish treated plating with its color fastness specified, evaluate the property by measuring the time until a discoloration occurs after dipping the sample into the ammonium sulfide solution (color fastness test).

(6) Corrosion resistance and adhesiveness of plating

To measure the corrosion resistance, perform testing such as a neutral salt spray test, a CASS test, or a sulfur dioxide test. To measure the adhesiveness, perform testing such as a tape test, a thermal-shock test, or a bend test.

(7) Residual salt on significant plating surfaces

If contamination by residual salts on the plating film is the issue, perform a test by a method agreed between the parties concerned.

(8) Other characteristics

The other characteristics of plating films include the followings: 1) solder wettability of plating; 2) electrical properties of plating; 3) plating hardness; 4) undercoat; 5) abrasion resistance. Depending on the agreement between the parties concerned, some of these properties may be requested.

(9) Plating name and symbol

Ep-Cu/Ni b, E-Ag 5 b

Bright industrial silver coating of 5 µm or more over bright nickel undercoat on a brass substrate

Ep-Cu/E-Ag 20/AT

Industrial silver plating of 20 µm or more with anti-tarnish chromate treatment on a brass substrate

Ep-Zn/Cu 10, Ni 5 b, E-Ag 5 b

Industrial bright silver coating of 5 µm or more over 10 µm copper undercoat and 5 µm bright nickel undercoat on a substrate made of zinc die cast alloy