Classification of corrosion protection methods (Part 1)

Classification of corrosion protection methods

	Active corrosion protection
	Passive corrosion protection
	Permanent corrosion protection
	Temporary corrosion protection

Active corrosion protection

The aim of active corrosion protection is to influence the reactions which proceed during corrosion, it being possible to control not only the package contents and the corrosive agent but also the reaction itself in such a manner that corrosion is avoided. Examples of such an approach are the development of corrosion-resistant alloys and the addition of inhibitors to the aggressive medium.

Passive corrosion protection

In passive corrosion protection, damage is prevented by mechanically isolating the package contents from the aggressive corrosive agents, for example by using protective layers, films or other coatings. However, this type of corrosion protection changes neither the general ability of the package contents to corrode, nor the aggressiveness of the corrosive agent and this is why this approach is known as passive corrosion protection. If the protective layer, film etc. is destroyed at any point, corrosion may occur within a very short time.


Permanent corrosion protection

The purpose of permanent corrosion protection methods is mainly to provide protection at the place of use. The stresses presented by climatic, biotic and chemical factors are relatively slight in this situation. Machines are located, for example, in factory sheds and are thus protected from extreme variations in temperature, which are frequently the cause of condensation. Examples of passive corrosion protection methods are:

	Tin plating
	Galvanization
	Coating
	Enameling
	Copper plating

Temporary corrosion protection

The stresses occurring during transport, handling and storage are much greater than those occurring at the place of use. Such stresses may be manifested, for example, as extreme variations in temperature, which result in a risk of condensation. Especially in maritime transport, the elevated salt content of the water and air in so-called seasalt aerosols may cause damage, as salts have a strongly corrosion-promoting action. The following are the main temporary corrosion protection methods:

	1. Protective coating method
	2. Desiccant method
	3. VCI method http://www.tis-gdv.de/tis_e/verpack/korrosio/schutz/schutz.htm

Leopad Group a leading provider of corrosion protection services ranges from the scope of blasting and painting, insulation, thermal spray application, passive fire protection, refractory and other services such as scaffolding, cable tray systems and cathode protection.

We are a Malaysian company with close to 3000 staff and over 10 offices and fabrication yards throughout the country. Leopad Group is dedicated to being the market leader for corrosion protection and provide the highest standards in the industry with the convenience of providing multi-disciplinary services through a single point of contact.

For further enquiries on our services, please contact our Business Development Department at +603-22600200 , website www.leopad.com or email at hq@leopad.com

Hot Facts About HDG (Hot Dip Galvanizing)

• Using zinc to protect steel from corrosion (hot-dip galvanizing) is a 150-year-old practice!

• Corrosion is caused by the inherent tendency of metals, when subjected to air and moisture, to revert to their original earthly forms, usually an ore state. They do this through a chemical or electrochemical reaction with the environment.

• Galvanizer’s kettles are set at temperatures ranging between 815 F and 850 F (435 C to 454 C).

• A galvanizer knows a piece of steel should be immersed for a specific amount of time in order for the metallurgical reaction between zinc and iron to reach completion. The completion of the metallurgical reaction is observed when bubbling of the molten zinc in the kettle stops. At this point, the galvanizing is complete and the steel is removed from the kettle to cool.

• Galvanizers can hot-dip galvanize a piece of steel that is larger than the kettle dimensions; it’s called progressive dipping.

• Zinc seals the underlying steel from contact with its environment. If the steel is exposed to the elements due to mechanical damage, the surrounding zinc corrodes sacrificially, protection the underlying steel from corrosive attack.

• The zinc coating on galvanized steel is uniform: inside, outside, corners and edges.

• The hot-dip galvanized reinforcing steel bond with concrete is at least as great as the bond of bare steel to concrete.

• When the Brooklyn Bridge was built, over 14,500 miles of hot-dip galvanized wire were used for its four main cables. Over 100 years later when the bridge underwent massive rehabilitation, the hot-dip galvanized wire was in excellent condition. Hot-dip galvanized steel lasts longer today than it did 20 years ago. Because of environmental laws, our air is cleaner and less contaminated with corrosive emissions.

• A reddish-brown staining infrequently develops on the surface of a newly galvanized piece of steel that is comprised entirely of intermetallic layers. The steel is not rusting; there is just a very small amount of iron in the zinc-iron alloy layers that is oxidizing, causing the staining to occur. This does not cause any adverse effects on the corrosion performance of the galvanized steel.

• Corrosion annually costs the US economy 3.2 percent of the gross national product, over $423 billion. Indirect costs to the public could raise the percentage as much as 5-11 percent. Some indirect costs of corrosion are: lost productivity due to traffic delays, accidents caused by corroded hand and guardrails, excessive use of nature’s raw materials and energy to replace corroded steel.

• Based on a study by NACE International (The Corrosion Society), members of Congress, and the Department of Transportation (DOT), better corrosion management can be achieved using preventive strategies at every level of involvement (owner, operator, user, government, Federal regulators, and general public).
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http://www.galvanizeit.org/hot-dip-galvanizing/what-is-hot-dip-galvanizing-hdg/facts-about-hdg

Leopad Group a leading provider of corrosion protection services ranges from the scope of blasting and painting, insulation, thermal spray application, passive fire protection, refractory and other services such as scaffolding, cable tray systems and cathode protection.

We are a Malaysian company with close to 3000 staff and over 10 offices and fabrication yards throughout the country. Leopad Group is dedicated to being the market leader for corrosion protection and provide the highest standards in the industry with the convenience of providing multi-disciplinary services through a single point of contact.

For further enquiries on our services, please contact our Business Development Department at +603-22600200 , website www.leopad.com or email at hq@leopad.com

Why Hot Dip Galvanizing in Corrosion Protection?

Corrosion Protection

Steel is an abundant, efficient building material that provides specifiers design freedom. However, for projects exposed to the atmosphere and other harsh environments, it is critical to coat the steel for corrosion protection. Often large construction projects target a 50-100 year design life, highlighting the need fordurable, long lasting corrosion protection.  There are countless examples demonstrating the proven protection of hot-dip galvanizing in some of the harshest environments.

The reason for the extensive use of hot-dip galvanizing (HDG) is the three-fold protective nature of the coating. As a barrier coating, it provides a tough, metallurgically-bonded zinc coating that completely covers the steel surface and seals the steel from the corrosive action of the environment. Additionally, zinc’s sacrificial behavior protects the steel, even where damage or a minor discontinuity in the coating occurs. Finally, the natural weathering of the coating results in the development of an additional layer of protection on the surface.

Barrier Protection

Barrier protection is perhaps the oldest and most widely used method of corrosion protection. It acts by isolating the base metal from the environment. Like paints, the hot-dip galvanized coating provides barrier protection to steel.  As long as the barrier is intact, the steel is protected and corrosion will not occur. However, if the barrier is breached, corrosion will begin.

Because a barrier must remain intact to provide corrosion resistance, two important properties of barrier protection are adhesion to the base metal and abrasion resistance. The tightly-bonded, impervious nature of zinc makes galvanizing a very good barrier coating.Furthermore, zinc corrodes approximately 1/10 to 1/40 the rate of steel depending on the environment, making the corrosion rate of a thin zinc coating equivalent to a much thicker steel piece. Coatings such as paint that have pin holes are susceptible to penetration by elements causing underfilm corrosion to spread rapidly.

Cathodic Protection

The Galvanic Series of Metals (right) lists metals and alloys in decreasing order of electrical activity

Cathodic protection is a more effective method of resisting corrosion.  It requires changing an element of the corrosion circuit by introducing a new corrosion element, thus ensuring the base metal becomes the cathodic element of the circuit.

Hot-dip galvanizing protects steel cathodically similarly to the sacrificial anode method.  Basically, a metal (zinc) anodic to the base metal (steel) is placed in the circuit to corrode in place of the base metal.  The Galvanic Series of Metals is a list of metals arranged in order of electrochemical activity in seawater (the electrolyte). This arrangement of metals determines what metal will be the anode and cathode when the two are put in an electrolytic cell. Metals higher on the list are anodic to the metals below meaning they provide cathodic or sacrificial protection when the two are connected.

From the list, it is apparent in the case of hot-dip galvanizing, zinc will protect preferentially corrode to protect the underlying base steel.  In fact, thecathodic protection of HDG ensures even if the coating is damaged to the point bare steel is exposed (up to ¼ inch in diameter), no corrosion will begin until all the surrounding zinc is consumed.

The zinc of the hot-dip galvanized coating will sacrifice itself to protect the underlying base steel until all the surrounding zinc is consumed.

Zinc Patina

Different than barrier and cathodic protection which resist corrosion of the steel itself, the zinc patina protects the zinc coating.  Zinc, like all metals, begins to corrode when exposed to the atmosphere.  Thus, freshly galvanized steel progresses through a natural weathering process when exposed to wet and dry cycles in the environment.

As galvanized coatings are exposed to free flowing air, the formation of the zinc patina begins with a thin layer of zinc oxides.  Then as the piece is exposed to moisture – rainfall, dew, humidity – the zinc oxide particles react with the water to form a porous, gelatinous zinc hydroxide.  Then during dry cycles, carbon dioxide reacts with the zinc hydroxide and converts to a thin, compact, and tightly adherent layer of zinc carbonate.  The rate of the patina formation varies according to the environmental conditions, but it typically takes approximately 6-12 months to fully develop.

The fully developed patina is a passive, stable film that adheres to the zinc surface and is not water soluble so it does not wash off in the rain or snow.  Because of this, the zinc patina corrodes very slowly and protects the galvanized coating underneath retarding the corrosion rate to about 1/30th the rate of steel in the same environment.  As the zinc patina develops, the galvanized coating will turn a matte gray color.

The zinc patina formation is critical in the long lasting corrosion resistance of hot-dip galvanizing. Because the patina development relies on natural wet and dry cycles found in the environment, the results of salt spray tests, which rely on constant wet exposure, are not accurate in predicting the life of galvanized coatings in the real world.

http://www.galvanizeit.org/hot-dip-galvanizing/why-specify-galvanizing/corrosion-protection

Leopad Group a leading provider of corrosion protection services ranges from the scope of blasting and painting, insulation, thermal spray application, passive fire protection, refractory and other services such as scaffolding, cable tray systems and cathode protection.

We are a Malaysian company with close to 3000 staff and over 10 offices and fabrication yards throughout the country. Leopad Group is dedicated to being the market leader for corrosion protection and provide the highest standards in the industry with the convenience of providing multi-disciplinary services through a single point of contact.

For further enquiries on our services, please contact our Business Development Department at +603-22600200 , website www.leopad.com or email at hq@leopad.com

Hot Dip Galvanizing HDG Process

The hot-dip galvanizing (HDG) process consists of three basic steps:

• Surface Preparation
• Galvanizing
• Inpection

Surface Preparation

Surface preparation is the most important step in the application of any coating.  In most  instances where a coating fails before the end of its expected service life, it is because of incorrect or inadequate surface preparation.  The galvanizing process has its own built-in means of quality control because zinc will not react with an unclean steel surface. Any failures or inadequacies in surface preparation will be immediately apparent when the steel is withdrawn from the zinc bath because the unclean areas will remain uncoated, and immediate corrective action can be taken.

Surface preparation for galvanizing consists of three steps:

Degreasing/Caustic Cleaning

A hot alkali solution, mild acidic bath, or biological cleaning bath removes organic contaminants such as dirt, paint markings, grease, and oil from the metal surface. Epoxies, vinyls, asphalt, or welding slag, which cannot be removed  by degreasing, must be removed before galvanizing by grit-blasting, sand-blasting,  or other mechanical means.

Pickling

A dilute solution of heated sulfuric acid or ambient hydrochloric acid removes mill scale and iron oxides (rust) from the steel surface. As an alternative to or in conjunction with pickling, this step can also be accomplished using abrasive cleaning or air blasting sand, metallic shot, or grit onto the steel. 

Fluxing

The final surface preparation step in the galvanizing process, a zinc ammonium chloride solution, serves two purposes. It removes any remaining oxides and deposits a protective layer on the steel to prevent any further oxides from forming on the surface prior to immersion in the molten zinc.

Galvanizing

During the true galvanizing step of the process, the material is completely immersed in a bath of molten zinc.  The bath chemistry is specified by ASTM B6, and requires at least 98% pure zinc maintained at  815-850 F (435-455 C).

While immersed in the kettle, the zinc reacts with the iron in the steel to forma series of metallurgically bonded zinc-iron intermetallic alloy layers, commonly topped by a layer of impact-resistant pure zinc.

Once the fabricated items’ coating growth is complete, it is withdrawn slowly from the galvanizing bath, and the excess zinc is removed by draining, vibrating, and/or centrifuging.

The metallurgical reaction will continue after the materials are withdrawn from the bath, as long as it remains near bath temperature. Galvanized articles are cooled either by immersion in a passivation solution or water or by being left in open air. 



Inspection

The inspection of hot-dip galvanized steel is simple and quick. The two properties of the hot-dip galvanized coating most closely scrutinized are coating thickness and appearance/surface condition. A variety of simple physical tests can be performed to determine thickness, uniformity, adherence, and appearance.

Products are galvanized according to long established, accepted, and approved standards of ASTM, the International Standards Organization (ISO), the Canadian Standards Association (CSA), and the American Association of State Highway and Transportation Officials (AASHTO).  These standards cover everything from the minimum coating thicknesses required for various categories of galvanized items to the composition of the zinc metal used in the process.

http://www.galvanizeit.org/hot-dip-galvanizing/what-is-hot-dip-galvanizing-hdg/hdg-process

Leopad Group a leading provider of corrosion protection services ranges from the scope of blasting and painting, insulation, thermal spray application, passive fire protection, refractory and other services such as scaffolding, cable tray systems and cathode protection.

We are a Malaysian company with close to 3000 staff and over 10 offices and fabrication yards throughout the country. Leopad Group is dedicated to being the market leader for corrosion protection and provide the highest standards in the industry with the convenience of providing multi-disciplinary services through a single point of contact.

For further enquiries on our services, please contact our Business Development Department at +603-22600200 , website www.leopad.com or email at hq@leopad.com