July 2013 Archives

Plate coating thickness management

The most importantly viewed aspect of plating coat quality control is the coating thickness management. Historically, troubles such as ending up with an 8μm thick coating despite 10μm zinc plate coating was what was ordered, etc. have been occurring endlessly. Here, let us take a look at how plate coating thickness is managed.

Principle of plating

(1) Principle of electroplating

In general, electroplating is performed as shown in the [Fig.1], where the product to be plated is made into an anode submerged in the plating bath, and cathode is the plate metal (i.e. nickel for nickel plating). A DC current is applied on them to plate. The following reaction is occurring at the poles.

At the cathode, the metal ions in the bath Mn+ (n is the valence of the metal ion, n=2 for nickel plating) acquires n electrons (e) becoming metal, and deposition on the product surface occurs. On the other hand, at the anode, the anode metal in emits n number of electrons and dissolves the bath, becoming metal ions （Mn+）.
By continuing to apply the current to the plating bath, this reaction takes place continuously and the plating thickness grows gradually. The plating process is halted when a desired plate thick is reached.
The important fact here is that the energy forming the plate coating is the "DC current" used for the reduction reaction of metal ions.
DC power is represented by Current (amps) x Time (min.), and the plating thickness is affected by the current and time per one product. This indicates that targeted plating thickness can be obtained in short amount of time by applying a large current, though it is not unlimited. There will be some limitations due to the plating bath types and shapes of the products to be plated.
In general, a concept of "Current density" is used to determine the strength of the electrical current. This indicates the strength of current applied per unit area of the product, and typically represented with A/dm² (Amperes per 1 square decimeter 100x100mm). Therefore, surface area of the product to be plated is calculated, and the applicable current density multiplied constitutes the current (A) to be applied per one product.

Nitriding is a process of forming a hard nitride material by diffusion penetrating nitrogen, and the use of ammonia gas was initially developed. The main purpose of the nitriding is to improve the wear resistance, but nitrocaburizing is aimed to improve the fatigue resistance.
Because nitriding and nitrocaburizing are processed at temperatures below the steel's transformation point, they are characterized for less thermal deformation caused than carburizing processed at temperatures above the steel's transformation point.
Nitriding processes are shown in [Table 1].

[Table 1] Types of Nitriding

(1) Gas nitriding

It is a method simply heating in ammonia gas that was initially developed. The ammonia contacting the heated steel surface is decomposed into activated atomic form nitrogen and hydrogen, then the nitrogen is diffused into the steel to form the nitride to harden.

(2) Plasma Nitriding (Ion Nitriding)

In a pressure reduced vacuum chamber, nitrogen and hydrogen are introduced where the furnace body is made into anode and the process target is made into cathode, and several hundred volts are applied to cause glow discharge at the anode side. The positively charged nitrogen ions and hydrogen ions collide into the workpiece that is cathodic. The collision raises the temperature of the workpiece and the nitride formed will diffuse into it.

(3) Gas Nitrocaburizing

Nitrocaburizing is a carbon diffusion process similar to nitriding process, and is aimed to improve fatigue resistance. Cracked gas of urea, ammonia and carbon dioxide added to nitrogen are used.

(4) Salt bath Nitrocarburizing

The Tufftride process developed by a German company Degussa Corporation is well known.

(5) Nitrosulphurizing

It is a process of also diffusing sulfur to give wear resistance and sliding properties.