How do you protect iron and steel from corrosion?
How do you galvanize?
Surface PreparationGalvanizing Inspection
What is the resulting metallurgical bond?
Service Life Expectations for Galvanized Steel
What is corrosion?
Corrosion is the reaction between a material and its environment that produces a deterioration of the material and alters its mechanical properties. The actual corrosion process that takes place on a piece of bare mild steel is very complex due to factors such as variations in the composition/structure of the steel, presence of impurities due to the higher instance of recycled steel, uneven internal stress, or exposure to a non-uniform environment.
It is very easy for microscopic areas of the exposed metal to become relatively anodic or cathodic. A large number of such areas can develop in a small section of the exposed metal. Further, it is highly possible that several different types of galvanic corrosion cells are present in the same small area of the actively corroding piece of steel.
As the corrosion process progresses, the electrolyte may change due to materials dissolving in or precipitating from the solution. Additionally, corrosion products might tend to build up on certain areas of the metal. These corrosion products do not occupy the same position in the given galvanic series as the metallic component of their constituent element. As time goes by, there may be a change in the location of relatively cathodic or anodic areas and previously uncorroded areas of the metal are attacked and corrode. This eventually will result in uniform corrosion of the area.
The rate at which metals corrode is controlled by factors such as electrical potential and resistance between anodic and cathodic areas, pH of the electrolyte, temperature and humidity.
How do you protect iron and steel from corrosion?
Barrier protection is perhaps the oldest and most widely used method of corrosion protection. It acts by isolating the metal from the electrolytes in the environment. Two important properties of barrier protection are adhesion to the base metal and abrasion resistance.
Cathodic protection is an equally important method for preventing corrosion. Cathodic protection requires changing an element of the corrosion circuit, introducing a new corrosion element, and ensuring that the base metal becomes the cathodic element of the circuit. Hot-dip galvanizing provides excellent barrier and cathodic protection. The sacrificial anode method, in which a metal or alloy that is anodic to the metal to be protected is placed in the circuit and becomes the anode. The protected metal becomes the cathode and does not corrode. The anode corrodes, thereby providing the desired sacrificial protection. In nearly all electrolytes encountered in everyday use, zinc is anodic to iron and steel. Thus, the galvanized coating provides cathodic corrosion protection as well as barrier protection.
How do you galvanize?
The galvanizing process consists of three basic steps: surface preparation, galvanizing and inspection.
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 due to incorrect or inadequate surface preparation.
The surface preparation step in the galvanizing process has its own built-in means of quality control in that zinc simply will not react with a steel surface that is not perfectly clean. Any failures or inadequacies in surface preparation will be immediately apparent when the steel is withdrawn from the molten zinc. Any areas that were not properly prepared will remain uncoated. Immediate corrective action is taken.
Surface preparation for galvanizing typically consists of three steps: caustic cleaning, acid pickling and fluxing.
Caustic Cleaning – A hot alkali solution often is used to remove organic contaminants such as dirt, paint markings, grease and oil from the metal surface. Epoxies, vinyls, asphalt or welding slag must be removed before galvanizing by grit-blasting, sandblasting or other mechanical means.
Pickling – Scale and rust normally are removed from the steel surface by pickling in a dilute solution of hot sulfuric acid or ambient temperature hydrochloric acid.
Fluxing – Fluxing is the final surface preparation step in the galvanizing process. Fluxing removes oxides and prevents further oxides from forming on the surface of the metal prior to galvanizing and promotes bonding of the zinc to the steel or iron surface. The method for applying the flux depends upon whether the particular galvanizing plant uses the wet or dry galvanizing process.
In the dry galvanizing process, the steel or iron materials are dipped or pre-fluxed in an aqueous solution of zinc ammonium chloride. The material is then thoroughly dried prior to immersion in molten zinc.
Galvanizing
In this step, the material is completely immersed in a bath consisting of a minimum 98% pure molten zinc. The bath chemistry is specified by the American Society for Testing and Materials (ASTM) in Specification B 6. The bath temperature is maintained at about 850 F (454 C).
Fabricated items are immersed in the bath long enough to reach bath temperature. The articles are withdrawn slowly from the galvanizing bath and the excess zinc is removed by draining, vibrating and/or centrifuging.
The chemical reactions that result in the formation and structure of the galvanized coating continue after the articles are withdrawn from the bath as long as these articles are near the bath temperature. The articles are cooled in either water or ambient air immediately after withdrawal from the bath.
Inspection
The two properties of the hot-dip galvanized coating that are closely scrutinized after galvanizing are coating thickness and coating appearance. A variety of simple physical and laboratory tests may be performed to determine thickness, uniformity, adherence and appearance.
Products are galvanized according to long-established, well-accepted and approved standards of the ASTM, the Canadian Standards Association (CSA), and the American Association of State Highway and Transportation Officials (AASHTO). These standards cover everything from the minimum required coating thicknesses for various categories of galvanized items to the composition of the zinc metal used in the process.
What is the resulting metallurgical bond?
Galvanizing forms a metallurgical bond between the zinc and the underlying steel or iron, creating a barrier that is part of the metal itself. During galvanizing the molten zinc reacts with the surface of the steel or iron article to form a series of zinc/iron alloy layers. The photomicrograph below shows a typical galvanized coating microstructure consisting of three alloy layers and a layer of pure metallic zinc. Moving from the underlying steel surface outward, these are:
The thin Gamma layer composed of an alloy that is 75% zinc and 25% iron,
The Delta layer composed of an alloy that is 90% zinc and 10% iron,
The Zeta layer composed of an alloy that is 94% zinc and 6% iron, and
The outer Eta layer that is composed of pure zinc.
Service Life Expectations for Galvanized Steel
The graph below is a plot of the thickness of the galvanized coating against the expected service life of the galvanized coating under outdoor exposure conditions. Most galvanized applications are 4 mils of thickness minimum per surface.
The expected service life is defined as the life until 5% of the surface is showing iron oxide (rust). At this stage, it is unlikely that the underlying steel or iron has been weakened or the integrity of the structures protected by the galvanized coating otherwise compromised through corrosion.
Graph – “Life of Protection”
Painting Over Hot-Dip Galvanized Steel
For years, protecting steel from corrosion typically involved either the use of hot dip galvanizing or some type of paint system. However, more and more corrosion specialists are utilizing both methods of corrosion protection in what is commonly referred to as a duplex system. A duplex system is simply painting or powder coating steel that has been hot dip galvanized after fabrication. When paint and galvanized steel are used together, the corrosion protection is superior to either protection system used alone.
Painting galvanized steel requires careful preparation and a good understanding of both painting and galvanizing. Many products have been galvanized and painted successfully for decades, including automobiles and utility towers. Past experience provides excellent historical data for how best to achieve good adhesion. By studying past adhesion failures and successes, galvanizers, paint companies, researchers, paint contractors, and other sources have created an ASTM specification detailing the process and procedures for preparing hot dip galvanized steel for painting. When the galvanized surface is prepared correctly, paint adhesion is excellent and the duplex system becomes an even more successful method of corrosion protection.
How a Duplex System Works
Before deciding how to protect steel from corrosion, it is important to understand how steel corrodes. Corrosion of steel takes place because of differences in electrical potential between small areas on the steel surface which become anodic and cathodic. When an electrolyte connects the anodes to the cathodes, a corrosion cell is created. Moisture in the air forming condensation on the steel surface is the most common electrolyte. In the electrolyte, a small electrical current begins to flow. The iron ions produced at the anode combine with the environment to form the loose, flaky iron oxide known as rust.In order to protect steel from corrosion, something must interfere with the corrosion cell, either by blocking the electrolyte or becoming the anode. Two common methods of corrosion protection are cathodic protection (the fonnation of another anode) and barrier protection (blocking the electrolyte from the steel surface). Hot dip galvanizing alone affords both types of protection.
Aging Characteristics of Galvanized Coatings
Galvanized steel can be divided into three categories: newly galvanized steel, partially weathered galvanized steel and fully weathered galvanized steel. Each type of galvanized steel must be prepared slightly different because the galvanized surface has different characteristics at each stage of weathering. It is important to know the age of the galvanized steel that will be painted.
Newly Galvanized Steel
Newly galvanized steel is zinccoated steel which has been hot dip galvanized after fabrication within the past 48 hours. The newly galvanized steel should not be water or chromate quenched, nor should it be oiled. This type of galvanized surface is typically very smooth and the surface may need to be slightly roughened, using one of the profiling methods described in this publication, to improve paint adhesion. A newly galvanized surface has little or no zinc oxides or zinc hydroxides, so no major cleaning is necessary.
Partially Weathered Galvanized Steel
Partially weathered galvanized steel has begun to form the protective zinc patina, but has not completed the process. Before painting partially weathered galvanized steel, it is important to know if the coating was chromate quenched. The presence of chromateconversion coatings can be determined by spot testing the galvanized steel according to ASTM B 201. If a chromate coating is detected the chromate layer must be removed, either by brushing off by abrasive blast cleaning, abrading the steel by sanding or allowing the steel to weather for six months.
Partially weathered galvanized steel should also be inspected for wet storage stain. Since wet storage stain is hygroscopic and has a larger volume than the zinc metal, paint adhesion can be seriously affected when this stain is painted. Wet storage stain is a whitish zinc corrosion byproduct. If the surface to be painted has wet storage stain, it should be carefully removed by brushing the stain with a mild ammonia solution, such as diluted household ammonia. Severe cases of wet storage stain should be brushed with a mild acidic solution, such as one part acetic acid mixed with 25 parts water. Follow these cleaning procedures with a clean, warm water rinse.
Partially weathered galvanized steel also should be slightly roughened to improve paint adhesion. Any of the surface profiling methods described in this publication can be used to prepare the surface.
Fully Weathered Galvanized Steel
Fully weathered galvanized steel has a completely formed zinc patina. The patina has a very stable and finely etched surface, which provides excellent paint adhesion. The only surface preparation needed is a warm water power wash to remove loose particles from the surface. In order to protect the surface, the power wash should not exceed 1450 psi. Allow the surface to completely dry before application of the paint system.
Profiling
In order to provide a good adhesion profile for the paint, the galvanized surface must be flat with no protrusions and slightly roughened to provide an anchor profile for the paint system. Filing high spots, sweep blasting, phosphating, and using wash primers or acrylic passivations are the most common methods of increasing the profile of a galvanized surface. Again, care must be taken not to damage the galvanized coating.
High Spots
Any high spots or rough edges should be removed and smoothed out in order to provide a level surface for paint. Use hand or power tools to grind down the high spots. Care should be taken to remove as little zinc as possible.
Sweep Blasting
In order to roughen the typically smooth galvanized surface after cleaning, an abrasive sweep or brush blast may be used. Care should be taken to prevent removing too much of the zinc coating. Particle size for a sweep blast of galvanized steel should range between 200 and 500 microns (8 to 20 mils). Aluminum/magnesium silicate has been used successfully in the sweep blasting of galvanized steel. Organic media such as corn cobs, walnut shells, corundum, limestone, and mineral sands with a Mobs hardness of five or less may also be used. The temperature of the galvanized part when blasting can have a Significant affect on the finished surface profile. Sweep blasting while the galvanized part is still warm, 175 to 390 degrees F, provides an excellent profile. Ambient conditions for sweep blasting are recommended to be less than 50 percent relative humidity and a minimum of 70 degrees F.
Repair of Damaged Galvanized Surfaces
Sometimes hot-dip galvanized coatings are damaged by excessively rough handling during shipping or erection. Welding or flame cutting may also result in coating damage.
When limited areas are damaged, the use of low melting-point zinc alloy repair rods or powders, the use of organic zinc-rich paints or metallizing is recommended to protect the area.
We would like to express our appreciation to The American Galvanizing Association for supplying material and photographs for our web site.
For additional information on the topics listed above please contact The American Galvanizing Association or Ben Pletcher, Sales Manager, Ohio Galvanizing Corp.
Specifications
The primary specification that government organizations, engineering firms, specifiers, and fabricators desire is ASTM A123. This specification is defined by the American Society for Testing and Materials. Items such as coating thickness, guidelines for acceptance, and testing methods are documented.
Additional specifications for areas such as the galvanizing of steel hardware and fasteners (ASTM A153) and the allowable practices for repair of galvanized steel (ASTM A7890) are also listed.
A 123
Standard for Specification of Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products
A 153
Standard for Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware
A 780
Standard for Specification for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings
Ohio Galvanizing is a member of the American Galvanizing Association and galvanizes all material to the ASTM A123 specification. We will provide the proper documentation for those jobs requiring certification to the specifications of ASTM A123.
With regards,
Kannan.
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