Corrosion protection:

Corrosion protection:

Corrosion protection is a change in a corrosion system that reduces corrosion damage.

 Corrosion protection systems:

Material

Environment

Phase limit

 

                       Changes in the corrosion system to prevent corrosion damage

 Definition:

The term corrosion protection designates all active or passive measures for protecting materials or

components from damage. Passive corrosion protection is the application of organic or inorganic

protective coatings. Active corrosion protection is the designation for all measures that exert a direct

influence on the corrosive medium (e.g. inhibitors in drinking water systems). Selection of the wrong

stainless steel quality can result in corrosion damage in specific cases. Even high-quality stainless

steels are not suitable for every application area! The effectiveness of a corrosion-protection  process     can be determined comparatively through salt spray testing.

Corrosion prevention techniques can be generally classified into                                       6 groups:

1.    Environmental Modifications

2.    Metal Selection and Surface Conditions

3.    Cathodic Protection

4.    Corrosion Inhibitors

5.    Coating

6.    Plating

(1)Environmental modifications:

Corrosion is caused through chemical interactions between metal and gases in the surrounding environment. By removing the metal from, or changing, the type of environment, metal deterioration can be immediately reduced.

This may be as simple as limiting contact with rain or seawater by storing metal materials indoors, or could be in the form of direct manipulation of the environmental affecting the metal.

Methods to reduce the sulfur, chloride or oxygen content in the surrounding environment can limit the speed of metal corrosion.

For example, feed water for water boilers can be treated with softeners or other chemical media to adjust the hardness, alkalinity or oxygen content in order to reduce corrosion on the interior of the unit.

(2)Metal selection and surface conditions:

No metal is immune to corrosion in all environments, but through monitoring and understanding the environmental conditions that are the cause of corrosion, changes to the type of metal being used can also lead to significant reductions in corrosion.

Metal corrosion resistance data can be used in combination with information on the environmental conditions to make decisions regarding the suitability of each metal.

The development of new alloys, designed to protect against corrosion in specific environments are constantly under production. Hastelloy® nickel alloys, Nirosta® steels and Timetal®titanium alloys are all examples of alloys designed for corrosion prevention.

Monitoring of surface conditions is also critical in protecting against metal deterioration from corrosion. Cracks, crevices or asperous surfaces, whether a result of operational requirements, wear and tear or manufacturing flaws, all can result in greater rates of corrosion.

Proper monitoring and the elimination of unnecessarily vulnerable surface conditions, along with taking steps to ensure that systems are designed to avoid reactive metal combinations and that corrosive agents are not used in the cleaning or maintenance of metal parts are all also part of effective corrosion reduction program.

(3)Cathodic protection:Galvanic corrosion occurs when two different metals are situated together in a corrosive electrolyte.

This a common problem for metals submerged together in seawater, but can also occur when two dissimilar metals are immersed in close proximity in moist soils. For these reasons, galvanic corrosion often attacks ship hulls, offshore rigs and oil and gas pipelines.

Cathodic protection works by converting unwanted anodic (active) sites on a metal's surface to cathodic (passive) sites through the application of an opposing current. This opposing current supplies free electrons and forces local anodes to be polarized to the potential of the local cathodes.

Forms of cathodic protection:

Cathodic protection can take two forms.

(i)Introduction  of  galvanic  anodes:

This method, known as a sacrificial system, uses metal anodes, introduced to the electrolytic environment, to sacrifice themselves (corrode) in order to protect the cathode.

While the metal needing protection can vary, sacrificial anodes are generally made of zinc,aluminum or magnesium, metals that have the most negative electro-potential.

The galvanic series provides a comparison of the different electro-potential - or nobility - of metals and alloys.

In a sacrificial system, metallic ions move from the anode to the cathode, which leads the anode to corrode more quickly than it otherwise would. As a result, the anode must regularly be replaced.

(ii)Impressed current protection:

A second method of cathodic protection is referred to as impressed current protection.

This method, which is often used to protect buried pipelines and ship hulls, requires an alternative source of direct electrical current to be supplied to the electrolyte.

The negative terminal of the current source is connected to the metal, while the positive terminal is attached to an auxiliary anode, which is added to complete the electrical circuit.

Unlike a galvanic (sacrificial) anode system, in an impressed current protection system, the auxiliary anode is not sacrificed.

(4)Corrosion inhibitors:

 Corrosion inhibitors are chemicals that react with the metal's surface or the environmental gases causing corrosion, thereby, interrupting the chemical reaction that causes corrosion.

Inhibitors can work by adsorbing themselves on the metal's surface and forming a protective film. These chemicals can be applied as a solution or as a protective coating via dispersion techniques.

The inhibitors process of slowing corrosion depends upon:

·         Changing the anodic or cathodic polarization behavior

·         Decreasing the diffusion of ions to the metal's surface

·         Increasing the electrical resistance of the metal's surface

Major end-use industries for corrosion inhibitors are petroleum refining, oil and gas exploration, chemical production and water treatment facilities.

The benefit of corrosion inhibitors is that they can be applied in-situ to metals as a corrective action to counter unexpected corrosion.

(5)Coatings:
Paints and other organic coatings are used to protect metals from the degradative effect of environmental gases.

Types of coatings:

Coatings are grouped by the type of polymer employed. Common organic coatings include:

·         Alykd and epoxy ester coatings that, when air dried, promote cross-link oxidation

·         Two-part urethane coatings

·         Both acrylic and epoxy polymer radiation curable coatings

·         Vinyl, acrylic or styrene polymer combination latex coatings

·         Water soluble coatings

·         High-solid coatings

·         Powder coatings

·         Zinc Nickel coating

(*)Zinc nickel coating:

Metallically clean steel parts are suspended as cathode (-pole) in an alkaline zinc-nickel bath

(+pole)

Through the supply of power the zinc-nickel alloy precipitates on the steel and forms a

protective layer

Advantages:

 Finishing of components to meet rigorous corrosion protection requirements

Particularly well-suited for finishing small parts and threaded parts

If there are scratches/cut edges, protection in certain dimensions is still provided (cathodic

protective effect - scratch end points make zinc ions available for protection), distances of

more than 2-3 mm should not be bridged

Resistance in accordance with salt spray test as specified in DIN 50021 up to 1,000 hours

Area of application: Implementation in outdoor areas.

(6)Plating:
Metallic coatings, or plating, can be applied to inhibit corrosion as well as provide aesthetic, decorative finishes.

Types of  plating:

There are four common types of metallic coatings:

1.    Electroplating:

 A thin layer of metal - often nickel, tin or chromium - is deposited on the substrate metal (generally steel) in an electrolytic bath. The electrolyte usually consists of a water solution containing salts of the metal to be deposited.

2.    Mechanical plating: Metal powder can be cold welded to a substrate metal by tumbling the part, along with the powder and glass beads, in a treated aqueous solution. Mechanical plating is often used to apply zinc or cadmium to small metal parts

3.    Electroless:

 A coating metal, such as cobalt or nickel, is deposited on the substrate metal using a chemical reaction in this non-electric plating method.

4.    Hot dipping:

  When immersed in a molten bath of the protective, coating metal a thin layer adheres to the substrate metal.

                                          

                             “THE END”