finishingcom

The home page of the finishing industry ®


WHY APPLY A COATING -- CORROSION ?
Mario S Pennisi
Principal Consultant - Penlia & Co
QUEENSLAND, AUSTRALIA

Introduction

Surface coatings such as powder coatings are applied to metallic surfaces to improve the aesthetic appearance of the item, and so that it functions better. We try to improve both the aesthetics and functionally by impeding the corrosion habit that is inherited by most commercial metals used in construction and fabrication.

Corrosion may be defined as a destructive phenomena, chemical or electrochemical, which affects the aesthetic appeal or an object; and in extreme cases may clause structural failure. The mechanism is based on anode and cathode reactions in an electrolyte. Corrosion takes place at the anode with the release of hydrogen gas or the formation of hydroxyl ions at the cathode. These hydroxyl ions may react with metal ions dissolved at the anode and form metal hydroxides or hydrated oxides. If these are insoluble they will deposit on the metal surface and may reduce the rate of corrosion.

The requirements for corrosion to proceed

For corrosion to proceed there must be an anode, a cathode and an electrolyte joined by an external current circuit.

Preventing corrosion

To prevent corrosion we have to break this triangle, by removing one of the legs.

The main technique available for reducing corrosion is to eliminate the electrolyte, either by:

Corrosion scientists achieve this by coating the metal with another metal which either

Types of corrosion

There are eleven main corrosion mechanisms:

General or uniform corrosion

Differences in electrical potential occur on the surface of a piece of metal due to small differences in chemical composition, phase differences, amount of cold work, etc. These differences set up small corrosion cells each with an anode and cathode. Corrosion continues until the metal is consumed or the film of rust formed on the surface sets up a barrier to the electrolyte.

Pitting corrosion

In pitting corrosion the surface of the metal is attacked in small-localised areas. Organisms in water or breaks in a passive film can initiate corrosion. Halides such as chlorides -- the main constituent of common salt, fluoride, etc stimulate pitting. In pitting corrosion very little metal is removed from the surface but the effect is marked.

Stress corrosion cracking

Failure is due to the simultaneous influence of static tensile stresses and a corrosive environment and this is specific to a particular metal. The stresses may be internal such as those caused by cold work, welding, heat treatment or external forces caused by mechanical stresses set up by assembly practices. A good example of this form of corrosion is 316 stainless steel in marine environments. 316 was developed to withstand attacks in chloride environments -- but if stressed the steel will fail by stress corrosion cracking.

Corrosion fatigue

Failure under repeated cycling stresses in a corrosive environment.

Intergranular corrosion

Corrosion occurs at the grain boundaries due to a difference in potential between the anodic grain boundaries and the cathodic grains. "Sensitised " stainless steels, where carbides have been precipitated in the grain boundaries during improper heat treatment or in the heat affected zone of a weld, are particularly susceptible to intergranular corrosion.

Filiform corrosion

Filiform corrosion appears as a network of corrosion trials, of a wormlike structure, particularly beneath thin organic coatings. Salts containing chlorides, which have been left on the surface prior to coating are suspected.

Crevice corrosion

Crevice corrosion occurs when there is a difference in ion, or oxygen, concentration between the metal and its surroundings. Oxygen starvation in an electrolyte at the bottom of a sharp V-section will set up an anodic site in the metal that then corrodes rapidly.

Galvanic or bi-metallic corrosion

Galvanic corrosion takes place between two different metals, or coatings, which are joined together in the presence of an electrolyte. Each metal has a potential different from any other metal when placed in an electrolyte. A series can be built up of all the metals relative to each other.

In seawater, the table is:

Anode end

Magnesium

Magnesium alloys

Zinc

Aluminium 5xxx series

Aluminium 3xxx series

Aluminium 1xxx series

Aluminium 6xxx series

Alclad

Cadmium

Aluminium 2xxx series

Mild steel -- low carbon steel

Wrought iron

Cathode end

Cast iron

410 Stainless steel -- active

50/50 lead-tin solder

304 Stainless steel - active

316 Stainless steel - active

Lead

Tin

Muntz metal

Manganese bronze

60/40 Brass

Nickel -- active

Aluminium bronze

Copper

Silicon bronze

Copper -- 30% nickel

Nickel -- passive

Stainless steel -- passive

Silver

Titanium

Graphite

Gold

Platinum

Voluminous oxides

The metals at the top of the table are more anodic than those below them and when in electrical contact in an electrolyte will corrode in preference to the metal below them on the table. The further apart the metals, the faster will be the corrosion rate.

Because of this relationship, zinc is applied to steel to protect it. When a holiday occurs in the zinc coating, the zinc will become the anode in the steel/zinc/electrolyte circuit and will corrode before the steel will. While zinc is available the steel will not corrode.

Fretting corrosion

Fretting corrosion occurs when two or more parts rub against each other. The rubbing action removes the corrosion products and exposes new metal to the electrolyte.

Erosion corrosion

Erosion is the removal of metal by the movement of fluids against the surface. The combination of erosion and corrosion can provide a severe rate of corrosion.

Selective leaching or demetalification

Demetalification is the removal of one of the alloying elements in an alloy by the electrolyte. This results in a "spongy" metal. Typical example is the removal of zinc in chloride waters from brass.

Incidence of corrosion

Evidence is available to show that the majority of metal failures due to corrosion occur through general, or uniform, modes. The next most common cause is stress corrosion cracking, followed by pitting corrosion and intergranular corrosion. These four modes account for about 80% of the failures examined. In this survey no failures due to galvanic corrosion were reported so the results are somewhat skewed.

Restricting corrosion

Corrosion can be retarded by any of a number of techniques. In some cases it is not feasible to eliminate even one of the three basic requirements for corrosion, ie an anode, a cathode and an electrolyte electrically connected to the electrodes.

Techniques available include:

Substrate preparation

When coatings are used as the means of reducing corrosion, it is essential that the coating adhere very tightly to the surface. For maximum adhesion, the substrate must be prepared correctly.

The Future

As it is unlikely that metal corrosion will go away the future for coating specialists is assured, whether it be metallic, organic or inorganic coating technology that is used.


Back to finishing.com Home Page