July 2010 Archives

"Localized Corrosion" is a term to describe a type of corrosion along with "Full Surface Corrosion", and indicates a type of corrosion where metal erosion occurs on localized areas as the term describes. The causes for that are thought to be (1) nonuniform metal composition, and (2) nonuniform corrosive environment.
A representative example for the case of nonuniform metal corrosion is "Contact Corrosion of Unequal Metals". For example, when a small piece of copper is placed on a steel plate and exposed to a corrosive environment, the copper hardly corrodes but a concentrated corrosion occurs on steel where contacting the piece of copper.
Additionally, when a welded steel is left submerged in water, corrosion occurs at weldments. This occurs because the metal property of weldments has changed from the base steel due to the welding process. This is a type of Contact Corrosion of Unequal Metals, but since the metal type is the same, it is called "Selective Corrosion of Weldments".

A representative example of Nonuniform Environment Corrosion is a case for bulkhead steel plates for shoreline protection. The waterline areas of these steel plates are severely corroded whereas the other parts of the plates are not. This is because the waterline areas are submerged during the high tides, and exposed to the air during the low tides. It can be said that the waterline areas and the other areas of the steel plates are subjected to nonuniform corrosive environments. In water, the differences in dissolved oxygen levels cause localized corrosion.

Localized corrosion also causes holes. A small opening diameter but deep corrosion is called "Pitting". For carbon steels, wide opening but shallow "dish or mortar shaped" corrosion occur often. For passive surfaced metals such as stainless steel, pitting occurs. The reason for that is thought to be the following. Passivated metals have several nanometers thick of highly corrosion resistant passive layer on the surface, and normally do not corrode in air and water. But if enough chlorine ions exist in the environment (i.e. sea water) this passive layer can be broken and nonuniform conditions will promote the advancing of the corrosion, thus pitting occurring.

Typically, metal corrosion becomes socially problematic not in harsh acidic or alkali environments but in human habitable atmospheric (humidity), temperature, ravely, rarely corrosive gases, airborne dusts, dissolved Oxyzen in water, free chlorine for disinfection, and by water dissolved elements such as Ca and Mg.

Let us think about metal corrosion in some general terms. Suppose We throw a piece of Polished metal in a corrosive environment. Depending on the metal type and the corrosive environment, the following changes would occur on the metal surface.

(1) No change on the surface

This means that no byproduct of corrosion was formed on the metal surface, or the surface deposits were in nanometer order thin, and the corrosion would not advance any further.

(2) Discolors but no further change

The corrosion byproduct has grown to become visible to the eyes, covering the surface densely but the corrosion will not progress further. The discoloration is of tens of nanometer thickness.

(3) Rough rusts are generated and the corrosion advances

The corrosion byproducts do not adhere tightly to the surface, causing the surface to be constantly exposed to the corrosion causing environment promoting the corrosion progress. The generated rust flakes off. This is the corrosion symptom for mild steel objects exposed to outdoor rain and wind. For coastal areas, salts would cause more severe rusts.

(4) No rusts evident but the corrosion advances

When the corrosion byproducts dissolve into the environment, the rust will not be visible but the corrosion will continue to progress and the metal will continue to be depleted. This is the case of metals in acids.

As seen above, there are different types of corrosion progression. Corrosion types can be categorized into:General Corrosion where the corrosion occurs on the full surface uniformly, and Local Corrosion where corrosion occurs in regional concentrations.
However, in a strict aspect, uniform corrosion cannot occur unless the Metal composition and structure are uniform and the environment is also uniformly corrosive. In reality, some localization of corrosion is occurring.

Corrosion of steel in water or high temperature high temperature gases occur almost uniformly, and are called General Corrosion, in an approximation term. The index used to indicate the magnitude of uniform corrosion are "Corrosivity" 25mg/dm²/day (25mg of corrosion per 1dm² per day, and 0.12mm/y (0.12mm thick corrosion per year).

Metal Corrosion and Corrosion and corrosion protection

　From this vol. forward, let us think about metal corrosions and protections.

It is well know that metals corrode in air, water, and in the ground causing gas/water leaks as well as playground structure collapses resulting in human injuries. Not all cases may result in serious accidents, but incidents such as corroded jewelry and railings are quite numerous.
It is quite logical to think that the metals, originally contained within the earth in forms of stable oxides and sulfides were mined and highly refined, will want to revert back to their original stable form if exposed to water rich in dissolved oxygen and chlorine ions, and air with 21% oxygen by volume. In order to prevent this and maintain the original functional purposes, alloying of metals, Plating for corrosion protection, and anti-corrosive painting processes are performed.

(1)Metal activation Series

Let us take a look at corrosiveness and non-corrosiveness of metals. In reactions of metals and acids, the metals with lower ionization energy than hydrogen will generate hydrogen. An order of metals based on reactiveness is called Metal activation Series, and is shown in Table 1.

[Table 1] Metal activation Series

The metals shown near the top are active (corrosion prone) metals, and they react with acids and generate hydrogen. The metals Cu and below only react when oxidants are present, and further Pt and below do not react even when oxidants are present. As can be seen above, metals have different levels of ionization energy, and with different levels of corrosion tendencies.

However, the actual corrosion phenomena are not so simple since the environmental variations affect the corrosion processes. For example, if the initial corrosion creates a an oxidized surface layer dense and stable sufficient in shielding the inner metal from the environment, no further oxidation would proceed. Since corrosion processes are largely affected by the surrounding environment, discussion the subject of corrosion without environmental concerns would be meaningless.

Chemical and electrolytic etching have been in use as the fundamental technology in all areas of applications in electronics, precision machinery, and printing industries. In some application areas etching has been found obsolete by the emergence of more advanced technologies, but etching still remains to be an important processing technology. Some examples are introduced below.

(3) Production of precision machine parts by etching

Most precision machine parts are produced by mechanical machining processes, but etching is more advantageous for the parts that are hard to produce, difficult to obtain required accuracies, small prototype production lots, and cost intensive parts due to expensive molds required.

In most cases, metal plates in thickness of 0.1~1.6mm are used for precision parts production by etching. The material can be SUS304, permalloy, silicon steel plates, and SUS316 as required for each application.

An outer blade section for electric razors is a familiar example for etch created products. These are made of 50~70μm thick SUS430 (13Cr) plates, and some techniques to improve the cutting performances were devised into the process. For instance, the holes are created by etching from one side first, then a press is used to make the hole edges sharp. Another is called "W etching" where slightly larger (10~20μm) pattern is deposited around the holes from the reverse side and hard chrome plating is deposited onto these narrow areas to improve the cutting performances. However, these methods are obsolete today, replaced by Electroforming techniques. The new methods utilize copper sided PCBs with 35~70μm copper plates. A negative pattern of the blade is exposure created on the copper, etching is used to remove the copper portion other than the blade pattern, then the blade is electroformed 50~79m thick onto this mold. The electroformed blade is removed from the mold and processed in a press to become the complete product.

Following are the application examples.

1) Measurements
Strain gauges, galvanometer mirror frames, electrical contacts and terminals, gaskets for meters, etc.

2) Electronics industry
Shadow masks for CRTs, high density electrical connectors, various metal grids, jigs for ceramic capacitors, magnetic recording heads, etc.

3) Optical instruments
Shutters/apertures/various slits for camera, etc.

4) Others
Precision micro nozzles, vibration plates, micro fine filters, bases for industrial diamond cutters, carriers for silicon wafer grinders, etc.