August 2012 Archives

Material types and difficulty of electroplating are explained.

[Table] Steel Alloys and Plating Difficulties

There are following types of surface treatments.

As opposed to the electrolytic polishing where the subject material is made an anode and electro-chemically etched, chemical polishing does not employ electricity. The chemical polishing obtains glossy surfaces by submerging the specimen metals and alloys in acidic or alkaline solutions with salts added.
This polishing method is not suitable in precision surface finishing since it cannot remove relatively large concaves and convexes, and is more suitable in attaining glossy finishes on surfaces by removing fine imperfections left after pre-polishing.

Therefore, chemical polishing is used for aesthetic surface treating and pre-treatments for other surface finishes, and is a very effective treatment method in terms of production efficiency and costs.

Chemical polishing has the following characteristics.
(1) Since it does not use DC currents such as electrolytic polishing, there are no issues such as electrical current distribution uniformity, and complex shaped objects can be polished evenly with relative ease.
(2) Operation is easy, and many items can be processed at one time.
(3) Oxidation is removed and the entire metal surface will be exposed after the polishing process.

The mechanism of chemical polishing is not clearly elucidated but two of the theories are introduced here. The first is regarding a purpose of obtaining flattening and smooth brightening and generally uses high viscosity solutions. When the metal submerged in chemical polishing solution is viewed microscopically, convex sections initially begin to dissolve and metal ions start to be diffused. The metal ion diffusion occurs more freely on convex sections compared to the concave sections, thus the metal ion concentration is higher around the concave sections so the dissolution of the metal is hindered. As a result flattening and smooth brightening occurs.
The other is regarding alternately occurring passivated layer formation and dissolution, causing microscopic polishing. In this case not much flattening will occur.

Electrical energy was used to promote eluting of the metals since the electrolyte alone cannot elute the metal. With chemical polishing, it is necessary to elute the metal by chemical effects alone, therefore the polishing solutions used for chemical polishing are comprised of strong acids, strong alkali, and strong oxidants.

Industrially, the chemical polishing is used on aluminum and its alloys, copper and its alloys, and stainless steels. For the chemical polishing solutions, mainly phosphoric acid based solution such as sulfuric acid - phosphoric acid type, phosphoric acid - nitric acid type, phosphoric acid - nitric acid - sulfuric acid type are used.
In order for the chemical polishing to be effective, the subject metal surface needs to be of a consistent structure where the dissolution rate is uniform. Otherwise a good result cannot be expected.

Electrolytic polishing is a process of providing flat and glossy surface on the subject material by applying electrical current between the material as an anode and another appropriate cathode submerged in an ca. The Fig.1 shows the electrolytic polishing system principle. The system is comprised of a DC power supply and its control device, an electrolysis tank and agitating device (the subject material is often rocked within the electrolyte solution), and a bath solution temperature control system (pre-heating the bath solution prior to the process, and cooling during the process).

There are various theories regarding the electrolytic polishing principle. Although the process is not yet precisely clarified, one of the theories explains that a highly viscous oxidation layer is formed on the surface by a reaction when the subject metal becomes anodic within the electrolyte solution. Boundary of this oxidation surface layer and the electrolyte is flat, so the oxidation layer on concave section of the substrate metal to be polished will be thicker, and the convex section would be less thick. The electrical current will be concentrated on the convex sections since electrolyte has small electrical resistance, but the oxidation layer has larger electrical resistance. Therefore, it is thought that the surface flattening occurs due to preferential dissolution of the minute convex sections.

It is difficult for the electrolytic polishing to achieve macro level surface flattening as shown in Fig.2, but is more suitable for micro level flat surfacing, in other words mirror gloss surfacing.

Mechanical polishing can flatten the surfaces by removing concaves and convexes, and macro and micro level flattening are possible by selecting appropriate abrasives. However, with mechanical polishing, surface layer crystals are destroyed and altered layers and hardened layers are formed. This causes microscopic visual blemishes due to micro polishing traces and embedded micro crystalline structures remaining on the surfaces. The electrolytic polishing, on the other hand, the surfaces are dissolved, not causing such altered layers on surfaces, leaving chemically clean surfaces with little blemishes.

However, there are relatively few types of metals that electrolytic polishing is applied, limited to Aluminum, Stainless Steel, Copper and its alloys. The reason for this is that the electrolytic polishing process is largely affected by the subject specimen's material purity, composition, construction, and heat treatment histories, as well as difficulties in the selection of electrolyte solutions and application conditions. The electrolyte baths are chosen based on the subject metal types, surface reflectance required. For Aluminum which is an amphoteric metal, either acidic or alkaline baths are used. In general, a bath solution mainly composed of phosphoric acid with sulfuric acid and chromic acid added were in common use, but a bath solution without phosphoric acid and chromic acid are now in use.