Posted 30 May 2007 18:56
Metals exposed to the elements rust due to the chemical reaction of the elements, water and oxygen, and corrosive chemicals such as salt.
Efforts to preserve the metal in its original state were made by coating them with paint, and one of the earliest electro-chemical means is electroplating. Since then, new methods were implemented with higher precision as to the position and thickness of the coating and with various materials and targets.Electroplating
Electroplating is the deposition of a metallic coating onto an object by putting a negative charge onto the object and immersing it into a solution which contains a salt of the metal to be deposited. The metallic ions of the salt carry a positive charge and are attracted to the part. When they reach it, the negatively charged part provides the electrons to "reduce" the positively charged ions to metallic form.
Anodizing, or anodising, is an electrolytic passivation process used to increase the thickness and density of the natural oxide layer on the surface of metal parts. This process is of no use on carbon steel because rust puffs up and flakes off, constantly exposing new metal to corrosion. But on many other metals, anodizing increases corrosion resistance and wear resistance, and provides better adhesion for paint primers and glues than bare metal. Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light. Anodization changes the microscopic texture of the surface and can change the crystal structure of the metal near the surface. Coatings are often porous, thick ones inevitably so, so a sealing process is often used to improve corrosion resistance. The process derives its name from the fact that the part to be treated forms the anode portion of an electrical circuit in this electrolytic process. Anodizing can prevent galling of threaded components. Anodic films are generally much stronger and more adherent than most paints and platings, making them less likely to **** and peel. Anodic films are most commonly applied to protect aluminium alloys, although processes also exist for titanium, zinc, magnesium, and niobium.
CATHODE-SPUTTERING.A typical cathode-sputtering system is illustrated in figure 1-16. This process is also performed in a vacuum. A potential of 2 to 5 kilovolts is applied between the anode and cathode (source material). This produces a GLOW DISCHARGE in the space between the electrodes. The rate at which atoms are SPUTTERED off the source material depends on the number of ions that strike it and the number of atoms ejected for each ion bombardment. The ejected atoms are deposited uniformly over all objects within the chamber. When the sputtering cycle is completed, the vacuum in the chamber is released and the wafers are removed. The masks are then removed from the wafers, leaving a deposit that forms the passive elements of the circuit, as shown in drawing at: