Monday, July 18, 2011

Corrosion Prevention


Introduction to Corrosion


Corrosion is a naturally occurring process, costing American industries approximately 6% of the entire United States GDP each year (PDF). The chemical interaction between a metal and its surroundings, corrosion degrades metal and leaves it prone to failure. Quite obviously, measures must be taken to manage corrosion so as to avoid failure.

While corrosion is a very broad problem, the solutions used tend to fall into a few common categories. Still, even within this narrowed range of solutions, a great deal of variation exists, requiring that methods and materials be standardized to ensure reliability. To this end, standard-developing organizations have brought together interested and educated parties to form a consensus on how to properly design, manufacture, test, handle and use products that are susceptible to corrosion. Chief among these are ASTM International, formerly the American Society for Testing and Materials, and NACE International, formerly the National Association of Corrosion Engineers.

Methods


Corrosion management comes down to a few key methods, each with their own advantages, disadvantages and variations. Frequently, several methods are used in tandem, leading to a particular blend of advantages and disadvantages that fit the requirements of the task at hand.

Prior to any specific method of corrosion prevention, the bigger picture of engineering design is brought into play. Some design decisions are more prone to corrosion, such as any place where two dissimilar metals are in electrical contact or when the shape of a structure allows liquids to collect. Because it isn’t always possible to avoid these pitfalls, additional care must be taken later to address them. Financial concerns are an issue too, as the simplest solution may simply be too expensive for the proposed application. Standards are especially important in this area as a guard against economic pressures incentivizing unsafe practices.

Leading in from engineering design, the first and most commonly used method of preventing corrosion revolves around the selection of materials. Simply picking a metal that is sufficiently resistant to corrosion is sometimes the easiest and most straightforward solution. Other times, the application requires certain characteristics, such as resistance to stress, that are not found in the metals most resistant to corrosion.

Filling these gaps, new materials, composed of two or more metals (alloys or intermetallics), are tailor made to fit all the performance characteristics required for certain applications, including resistance to corrosion. The most famous of these is stainless steel. A category of materials rather than one specific formulation, many distinct variations of stainless steel are used in a wide range of industries. Still, a single material might not exist that fulfills all the physical requirements at an acceptable price.

The second method involves placing a barrier between the vulnerable metal and the corrosive elements. In some cases, the early stages of corrosion naturally create a thin, protective layer over the material that impedes further damage. This is either allowed to happen naturally or stimulated chemically. Otherwise, the metal is manually covered by a layer of something more resistant to corrosion (metallic or not), such as the coat of rustproof paint on your car. This allows designs to use a material resistant to corrosion for the outer surface and a material with other desirable characteristics (resistance to stress or affordability) for the rest of the structure. One key problem is that if the covering corrodes all the way through at a single point, corrosion will then damage the underlying metal and its effects might be hidden by the rest of the covering.

The third method involves a sacrifice. A vulnerable structure such as a zinc anode is electrically connected to the primary metal, drawing away corrosive factors and corroding instead of the primary metal. One major use of this method is for storage tanks that hold liquids conducive to corrosion. The obvious drawback here is that the sacrificial structure is sacrificed and must be repaired or replaced.

Other methods are plentiful as well. It is sometimes possible to control the metal’s environment, removing corrosives or adding other substances to counteract their effects. Other times, a current can be run through metal to provide anodic protection so that galvanic corrosion is prevented. In some instances, parts can be made so that they are cheap, expendable and easy to replace, thrown out as corrosion overcomes them. Another option is to simply make the outer structure thicker by adding metal, a solution frequently used in situations where the environment is so corrosive that attempting to prevent it is futile and the additional mass and volume is not a problem. Lastly, there are cases where failure due to corrosion is unavoidable and a short useable life span is accepted as a part of the design.

Overview


Corrosion as a force is an ever present threat to our industries and products. Dealing with it is important. Dealing with it properly is even more important. Given the wide variety of corrosive situations, materials involved, possible solutions and the reality of economic pressures affecting decisions, the widespread use of standards is a necessary safeguard for everybody involved.

Corrosion

Defined generally as the breakdown of matter due to (electro)chemical interactions with its environment, corrosion affects every metal that humanity uses. These chemical interactions can then alter the physical properties of metal, affecting its reliability in a negative way. As a result, corroded metal can fail, potentially catastrophically.

Far from being the avoidable result of a narrow set of conditions, corrosion occurs readily in most environments. The most telling example of this is rust, the result of iron chemically bonding with oxygen in the presence of water. Since both water and oxygen are present in air, rust is possible whenever iron is in contact with air. In fact, most metals ordinarily undergo corrosion in their natural state. The few that do not react strongly in their natural state, such as gold or silver, are prized primarily for that quality. Because of metal’s natural tendency to corrode, it should always be a consideration when dealing with anything metallic.

Understanding Damage from Corrosion


Essentially, corrosion affects metal by modifying its chemical structure and, as a result, its physical properties. In the real world, this takes on many forms. If the products of corrosion remain attached to the metal, they result in added weight and volume, causing additional strain. If the products flake off of the initial piece of metal, then corrosion is free to continue deeper into the metal, slowly eating away at the whole. Other effects sometimes visible to the naked eye are the formation of cracks, pits or crevices that obviously degrade the metal’s structural integrity. Several other forms of corrosion, most notably hydrogen embrittlement, as well as intergranular and dealloying corrosion, attack the internal, molecular structure of metal without any clear external visible effects, weakening it so that ordinarily acceptable stresses can result in unexpected failure.

Preventing Damage from Corrosion


Because the chemical reactions that comprise corrosion are a result of the innate properties of metals, overcoming them is very difficult. Thus, many methods of dealing with corrosion seek to either redirect corrosive forces or to slow them down to an acceptable rate. Popular methods include covering the metal with a material more resistant to corrosion as a protective shield or placing a metal even more vulnerable to corrosion near the vital structure to draw away corrosive factors. Other methods involve the use of special materials, controlled environments or electrical currents.

What Happens When Corrosion is not Prevented


Corrosion damage can be relatively harmless, such as the rust residue left by the bottom of a can of shaving cream on your sink. Other times, corrosion damage leads to catastrophic failures, such as the collapse of the Silver Bridge, where 46 lives were lost and a major highway was closed. Alternatively, corrosion can compound poor design decisions, such as the Guadalajara sewer explosion, which caused over 200 deaths and left over 2,000 injured and 20,000 homeless.

Aside from the dangers of mechanical failures, corrosion also poses direct biological hazards. When pipes carrying drinking water suffer serious corrosion, potentially dangerous substances can leech into our water supply. On the other end of the scale, surgical implants and instruments have to be especially resilient or they risk harming the health of patients.

Corrosion’s Economic Impact


Because of the reliance of construction on metal and how vulnerable it is to corrosion, numerous studies have been carried out to gauge its financial impact. A lengthy nationwide study (PDF) completed in 2001 estimated that the direct cost of corrosion to the United States in 1998 was 279 Billion dollars, 3.2% of the U.S. GDP. Furthermore, the study said that indirect costs, much more difficult to accurately calculate, were likely to have been even higher, raising the total cost of corrosion to around 6% of the GDP. Some of this cost was found to have been avoidable, where existing technologies, materials, and methods were available to prevent the expense. More than half, however, was deemed unavoidable, a staggering expenditure with no known process for mitigation.

Standards, What They Are and Who Makes Them


Responding to the necessity of dealing with corrosion, several industry-wide organizations have gathered experts and interested parties together, publishing voluntary standards to address these concerns. Benefitting everybody, the use of standards promotes fair competition for producers and safe, reliable products for consumers. Key among standard-developing organizations involved in corrosion control are ASTM International, formerly the American Society for Testing and Materials, and NACE International, formerly the National Association of Corrosion Engineers.

ASTM standards include widely applicable corrosion tests, methods of measurement, data analysis, equipment calibration and information management, ensuring that the results of reliability tests are both accurate and precise. NACE standards include more specific applications, ranging from materials selection in H2S-containing environments in oil and gas productions (parts 2, 3), or cathodic anti-corrosion measures for prestressed concrete cylinder pipelines or steel water storage tanks, to the installation of thin metallic wallpaper lining in air pollution control equipment. It should however be noted that ASTM and NACE both publish standards detailing procedures from the widely applicable to the niche.

Continuing Forth


Accounting for corrosion plays an immense role in industries touching every aspect of our lives and cannot be taken lightly. Standards published by organizations such as ASTM or NACE represent the sum total of human knowledge when it comes to managing corrosion and serve as an indispensible resource. As such, the widespread use of standards promotes increased reliability and safety, translating into a higher quality of life for all.