Corrosion annually costs the U.S. economy 3.2 percent of the gross national product, over $279 billion. Indirect costs to the public could raise the percentage to as much as 6 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. The global impact of corrosion is almost immeasurable.
How does rust work?
Rust is the common name for a very common compound, iron oxide. Iron oxide, the chemical Fe2O3, is common because iron combines very readily with oxygen – so readily, in fact, that pure iron is only rarely found in nature. Iron (or steel) rusting is an example of corrosion – an electrochemical process involving an anode (a piece of metal that readily gives up electrons), an electrolyte (a liquid that helps electrons move) and a cathode (a piece of metal that readily accepts electrons). When a piece of metal corrodes, the electrolyte helps provide oxygen to the anode. As oxygen combines with the metal, electrons are liberated. When they flow through the electrolyte to the cathode, the metal of the anode disappears, swept away by the electrical flow or converted into metal cations in a form such as rust. For iron to become iron oxide, three things are required: iron, water and oxygen. Here’s what happens when the three get together.
When a drop of water hits an iron object, two things begin to happen almost immediately. First, the water, a good electrolyte, combines with carbon dioxide in the air to form a weak carbonic acid, an even better electrolyte. As the acid is formed and the iron dissolved, some of the water will begin to break down into its component pieces – hydrogen and oxygen. The free oxygen and dissolved iron bond into iron oxide, in the process freeing electrons. The electrons liberated from the anode portion of the iron flow to the cathode, which may be a piece of a metal less electrically reactive than iron, or another point on the piece of iron itself.
The chemical compounds found in liquids like acid rain, seawater and the salt-loaded spray from snow-belt roads make them better electrolytes than pure water, allowing their presence to speed the process of rusting on iron and other forms of corrosion on other metals.
Zinc comprises an estimated 0.004% of the Earth’s crust and ranks 25th in order of material abundance in the Earth. Zinc’s most remarkable quality is its natural capacity to protect. By protecting steel against corrosion, zinc protects buildings, automobiles, shipsand steel structures of every kind from corrosion by the atmosphere, water,and soil. By protecting against corrosionand its costly effects, zinc extends the life of steel, thus protecting investments. A typical galvanized coating can now be expected to last70 to 150years without maintenance in most urbanand rural atmospheres.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.
Zinc is 100% recyclable. Over 80% of the zinc available for recycling is currently recycled. More than one-third of the zinc consumed in North America is produced from recycled materials. Due to the long lifespan of most zinc-coated products like galvanized steel, which in some cases may last maintenance-free for over 100 years, much of the zinc produced in the past is still in use. Zinc makes the average automobile last longer-17 pounds of zinc protect it from rust, 20 pounds are used to make zinc die-cast parts like door handlesand locks,and each tire contains about 1/2 pound of zinc, needed to cure rubber.
History of hot dip galvanizing (HDG)…
The recorded history of galvanizing goes back to 1742 when a French chemist named P.J. Malouin, in a presentation to the French Royal Academy, described a method of coating iron by dipping it in molten zinc. In 1836, Stanilaus Tranquille Modeste Sorel, another French chemist, obtained a patent for a means of coating iron with zinc, after first cleaning it with 9% sulfuric acid and fluxing it with ammonium chloride. A British patent for a similar process was granted in 1837. By 1850, the British galvanizing industry was using 10,000 tons of zinc a year for the protection of steel. For over 150 years, hot-dip galvanizing has had a proven history of commercial success as a method of corrosion protection in myriad applications worldwide.
Zinc’s galvanic action…
Most zinc rich paints, epoxy paints and all other types of coatings merely serve as barrier coatings, when it comes to protecting iron and steel, as they are incapable of the electro-chemical ‘galvanic’ action which is the key to galvanic protection. As with most paint coatings, the quality of the application is a major factor in determining the long-term performance of the coating.
Zinc has a higher electro-motive potential than iron or steel and in the presence of an electrolyte (moisture) will be attacked to protect the steel. Zinc ions go into solution, liberating electrons which cause a current flow into the steel to prevent ferrous ions from going into solution and begin the electro-chemical corrosion cycle. To function anodically, the zinc particles must be in close contact with one another so that the film itself is conductive. 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, protecting the underlying steel from corrosive attack. This is not possible with barrier coatings which, once breached, will allow the corrosion to spread through a process referred to as creepage.
With the economic solution and unrivaled barrier and cathodic protection from corrosion that galvanizing steel provides, it’s no wonder it is used in thousands of applications throughout the world. Almost any steel that will be exposed to the elements in some fashion (directly or indirectly) is a prime candidate for galvanizing.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 galvanizing…
Though the process may vary slightly from plant to plant, the fundamental steps in the galvanizing process are:
Soiland grease removal: A hot alkaline solution removes dirt, oil, grease, shop oil, and soluble markings. Pickling – Dilute solutions of either hydrochloric or sulfuric acid remove surface rust and mill scale to provide a chemically clean metallic surface.
Fluxing: Steel is immersed in liquid flux (usually a zinc ammonium chloride solution) to remove oxides and to prevent oxidation prior to dipping into the molten zinc bath. In the dry galvanizing process, the item is separately dipped in a liquid flux bath, removed, allowed to dry, and then galvanized. In the wet galvanizing process, the flux floats atop the molten zinc and the item passes through the flux immediately prior to galvanizing.
Galvanizing: The article is immersed in a bath of molten zinc at between 815°- 850°F (435°- 455°C). During galvanizing, the zinc metallurgically bonds to the steel, creating a series of highly abrasion-resistant zinc-iron alloy layers, commonly topped by a layer of impact-resistant pure zinc.
Finishing: After the steel is withdrawn from the galvanizing bath, excess zinc is removed by draining, vibrating or – for small items – centrifuging. The galvanized item is then air-cooled or quenched in liquid.
Inspection: Coating-thickness and surface-condition inspections complete the process.
Hot dip galvanizing – Some draw backs…
When all is said and done, it is irrefutable that galvanizing is the pre-imminent method for long-term protection of iron and steel and that hot-dip galvanizing is the most recognized and common method of achieving this. There is also no denying the fact that working with and around molten zinc is a noxious, dangerous and bio-hazardous process. Along with the very obvious, there are other physical short comings worth noting.
- Hot dip galvanizing can only be done in a galvanizing plant. Site application is not possible.
- Costly transportation to and from the galvanizing plant, and the ensuing downtime.
- The dimensions of the component or structure are limited by the size of the zinc bath.
- Objects being hot-dipped are exposed to embrittlement and possible warpage.
- Because of the coatings hardness, it has a tendency to flake off when subjected to hard impacts or sharp bends.
- Zinc coating may tend to be of uneven thickness due to variance in the cross section of the steel.
- Coating defects such as bubbles, irregular films, etc. are simply inevitable and the same cannot necessarily be cured by re-dipping, both in terms of cost and impracticality.
- Weld joints have to be treated with extra care to prevent them from becoming corrosion centers.
- Hot-dip galvanized surfaces cannot be easily top coated without proper surface preparation.
Although there are some other inconveniences related to hot dip galvanizing the above items refer to the more common and significant issues.
The ZRC Advantage…
ZRC is the only cold galvanizing compound on the global market that is ISO 9001 registered and recognized by independent test laboratories as an equivalent of hot-dip galvanizing. ZRC is a recognized Component under the Component program of Underwriters Laboratories, Inc. This is achieved by the high zinc loading (95% by weight in the dry film), mixed in a special binder that does not insulate the zinc particles either from each other or from the base substrate.
Initially, the protection provided by ZRC is wholly that of galvanic action. As the zinc is sacrificed, zinc corrosion products are formed through the reaction of zinc ions with moisture and carbon dioxide. Zinc hydroxycarbonates and other zinc salts form in the film making the coating denser and reducing its conductivity. Thus the anodic action of the zinc continues until the film is converted into a dense, impervious barrier, resistant to weather, water and fume attack. However, if the coating is damaged, fresh zinc metal is readily available to provide renewed galvanic action. Thus, the protection afforded by ZRC is twofold; cathodic protection and barrier protection. Essentially, it is a self-healing film as any damage to the barrier initiates renewed galvanic action.
ZRC Cold Galvanizing Compounds have been around for more than 50 years. Major projects around the world, such as the ‘Peace Bridge’ over the Niagara River in N. America and the Aomori Cement Plant in Japan, are proof of the trust and respect for ZRC cold galvanizing compounds. ZRC Cold Galvanizing compounds can be applied via dipping, spraying or brushing and best of all it can be done at the factory or the job site. This is not possible with hot dip galvanizing. To add profit to efficiency, cost comparisons in UAE, one of the world’s busiest construction areas (Dubai alone is utilizing 25% of the world’s construction cranes), ZRC provided a savings of 65% over hot dip in material and application alone.
ZRC provides all of the advantages of hot dip galvanizing while overcoming the earlier outlined drawbacks associated with hot dip. Surfaces that need to be safeguarded with ZRC need not be pristine. They must be free of oils, greases, debris and loose rust. ZRC compounds also act as top notch primers eliminating the need for special preparations prior to top coating. ZRC compounds readily lend themselves to powder coating. Needless to say, in all instances, prescribed procedures must be followed for best results.
Besides the ISO 9001 registration and participation in the Underwriter’s Laboratories’ Component Testing Program MH7035 with retesting every year, some of the other specifications that ZRC meets and exceeds are:
- Department of Defense galvanizing repair spec – DOD-P-21035A
- USAF Zinc-dust spec -MIL-P26915A
- Preece Test for hot-dip galvanizing- ASTM Des A239
- 3000 hours of salt spray testing without failure – ASTM-B117-73
- Canadian Government Spec for hot-dip repair- 1-GP-181A
- Meets and exceeds “Standard Practice for Repair of – ASTM Des. A-780
- Damaged Hot Dip Galvanized Coatings” Authorized for food contact under US Federal Regulation – 21 CFR 175.390
- May be used as a coating on bulk re-usable food containers
Meets and exceeds SSPC-Paint 20- Spec for zinc-rich primers
- Meets and exceeds SSPC-Paint 29- Spec for zinc dust sacrificial primer
ZRC compound is chemically accepted by the US Dept. of Agriculture for use in processing or storage areas for meat and poultry food products.
ZRC provides longer lasting protection…
Like galvanizing, films of ZRC afford protection to steel substrates electrically. Applied to a steel substrate the zinc film completes an electrochemical cell with the substrate in which the zinc metal of the ZRC film becomes the anode of the couple, and thereby ensures that, in water or a corrosive environment, the steel will behave entirely cathodically. As such, while the zinc primer maintains contact with the steel, the steel cannot oxidize, and all corrosion is confined to the film of ZRC.
As the potential difference between the zinc and the steel is fixed (more or less), the overall resistance of the cell (R) most significantly determines the amount of corrosion. This resistance is made up of the individual resistances of the various components of the cell. These include the metallic resistances within the zinc film and within the steel substrate, the electrolytic resistance of the electrolyte, and the resistance of any corrosion product that may build up on either the steel or the zinc.
Hot dipped galvanized films are made up of solid zinc metal, melted onto the steel. In these films, the resistances are, like those of the steel itself, virtually zero. The electrical resistivity of zinc metal is 5.5 microhms/cm-1. In pure zinc/steel couples, such as these, current flows readily (so that zinc corrosion takes place rapidly). It is for this reason, and the high efficiency of salt water as an electrolyte, that hot dip galvanizing films corrode rapidly in seawater until the insulative effects of corrosion product add resistance to the cell.
In films of ZRC, the zinc is present in the form of discrete spheres of zinc dust which are packed close enough together to ensure a virtual tangential contact between the zinc particles and the steel substrate. True contact in ZRC is compromised by the presence of an ultra thin sheath or monomolecular layer of polymeric binder or glue which is necessary to sustain the cohesive integrity of the film and ensure that the film sticks to the steel. The electrical corrosion current must conduct through this insulative sheath.
The presence of this binder sheath, therefore, decreases the conductivity of the ZRC film significantly compared to films of hot dipped galvanizing. Films of dry ZRC have much higher resistivity (the reciprocal of conductivity). Laboratory determined values have approached 2.5 x 106 ohm/cm. This reduced conductivity does not impede the protective properties of the film, for as long as enough current can continue to flow through the couple toward the zinc (i.e. as long as current is not discharged from the steel), the steel will remain galvanically protected. In fact, in aggressive environments such as seawater the ZRC film itself corrodes more slowly (compared to pure zinc) and will, therefore maintain protection for a longer duration. Only when the resistance of the cell becomes so high from corrosion bi-product that current ceases to flow between cathode and anode will cathodic protection be nullified.
The foregoing data on corrosion and galvanizing is assembled from published material available on the internet and technical data published by ZRC Worldwide, Inc. The purpose is to give the reader an honest, reasoned and well rounded understanding of prior and existing art related to galvanic protection of steel. Millions of tons of steel are galvanized annually around the globe and there can be no question about the viability of the process. From the data provided, a strong case can be made for ZRC Cold Galvanizing Compound as a supplemental product not only because of its proven galvanic action but also because of beneficial features that are absent in the hot dip galvanizing process. As with many other processes, having additional modalities creates greater overall business opportunities. During market research, it was discovered that galvanizing projects were languishing because galvanizing plants were overbooked with orders. Project owners had no choice but to wait due to lack of other galvanizing options. Unlike hot dip galvanizing, ZRC application does not require factories, heavy equipment, specific skills and a large staff. A modest investment in paint brushes, or spraying equipment and a steady hand is all that is needed to provide dependable and long lasting protection every bit as good as hot dip galvanizing, and then some. ZRC to the rescue!!!
V. Shankar, Director, Anasuyaa Metal Process & Engineering Company Pvt. Ltd. Mumbai.