Warning! The air you are breathing could be harmful…. to your compressed air system. Sound strange? Some gases found in the air used within your compressed air system can be safe to humans but very damaging to metal parts, and can easily be present without plant operators aware of their existence. And, in fact, corrosive gases in the air stream can cause serious damage to the operating parts of compressed air systems, leading to unexpected breakdowns that could require long repair periods and expensive replacement parts.

To help make sense of this issue, we will review what certain corrosive gases can do to compressed air systems, identify ways to determine if the compressed air is contaminated with corrosive gas and provide a course of action to help minimize, or in some cases, eliminate potential damages.

Before considering the various ways to remove corrosive gases from the air, being aware of their presence and monitoring the effectiveness of the removal system is critical. To begin, there are two basic ways to monitor the corrosive properties of the air entering the system: measure the concentrations of gases known to be corrosive or measure the effects of those corrosive gases.

Once a quantitative measurement of the corrosive potential of the air is obtained, the magnitude of the corrosion problem must be determined.

Measuring Corrosive Properties of the Air

The biggest challenge when dealing with corrosive gases is determining whether or not the gases are present in the first place. Monitoring the corrosive properties of the incoming ambient air can help determine how to avoid these problems.

Pinpointing the corrosion level of the air is critical to determining the magnitude of the problem. In addition, once steps have been taken to protect the compressed air system from these corrosive gases, monitoring is needed to ensure that the system is functioning correctly.

There are two methods that have been used to quantify the corrosive properties of gaseous mixtures. The first method is the direct measurement of chemical components in the air. When performing a chemical analysis of the components of the air, a sample must be taken and sent to a laboratory for analysis or, in some cases, the analysis can be performed on site in a mobile testing facility. Companies that sell corrosive gas protection systems or independent testing services can offer an extensive array of tests.

There are several drawbacks, however, of laboratory analyses. First of all, the testing will only reveal the chemicals that the laboratory personnel have been asked to identify. Secondly, a list of the chemicals in the air and their concentrations may not be the best indicator of the corrosive nature of the environment. In many cases, certain corrosive compounds can act in concert to increase or decrease the rate of corrosion of the individual compounds. In addition, the concentrations of harmful gases can also rise and fall. Therefore, occasional testing may not reveal the full extent of the problem.

The second method for measuring the effects of corrosive gases is reactivity testing. This method is more cost-effective than laboratory testing, which can cost six to seven times more than reactivity testing, and could offer a more direct way of quantifying the problem. A simple pH test on a condensate sample can indicate the presence of corrosive gases in the airstream. A pH of 7 is neutral. If the pH is less than 7, the condensate is acidic and it will attack copper and steel. If the pH is greater than 7, the condensate is basic and it will attack metals such as aluminum. This test will simply indicate the presence of a corrosive compound that will dissolve in water to form an acid or a base. Compounds that do not form acids or bases when mixed with water will not change the pH of the condensate.

While this test will indicate whether or not there is a problem, it will not identify the extent of the problem. If the pH test does indicate a problem, another testing method will need to be used to determine the extent of the problem.

In this case, one of the best ways to quantify the effects of corrosion is to measure the reaction of the corrosive compounds in the air on a test specimen. The simplest way to do this is to hang a metal strip in the air stream, usually in the particulate filter housing, for a period of time. The strip then is sent to a laboratory for analysis.

As corrosive gases react with the base metal of an air compressor component, a chemical product in the form of a film will build up on the surface of the base metal. Specially prepared copper and silver strips are usually exposed for a time period varying from 30 to 90 days. Analysis of the test strip will reveal the average film thickness built up during the test period.

There are two testing methods that will indicate film thickness. The mass gain of the metal strip can be directly correlated to the average corrosion film thickness. Precise measuring of such a small mass gain can be difficult, however.

Battelle Laboratories and Bell Telephone Systems researched an electrolytic reduction method, where the exposed metal strip is placed in an electrolyte solution and an electrical current is passed through the solution from the test strip to a platinum anode. The voltage difference between the test strip and the platinum strip will change as corrosion film is reduced. The time it takes for the film to be reduced can be correlated to the average film thickness. This analysis will report the average rate of corrosion film buildup for the period of time that the strips were installed. Unfortunately, exposure to corrosive gases usually does not occur at a constant rate. But knowing the rate of corrosion at different times of the day or week can sometimes indicate the source of the corrosive gases.

While using the corrosion film thickness on a test strip as an indicator will provide good results, the frequency of testing can be a detrimental factor. The solution is to provide a way to measure the mass gain of a test specimen at a smaller time interval so that a sudden increase in the concentration of corrosive chemicals in the air can be addressed.

One way to measure mass gain in a compact package is to provide an exciting force to the test sample, causing it to vibrate, such as quartz crystal microbalance. The first natural frequency is dependent upon the mass of the sample. As mass increases, the frequency will decrease. If the frequency can be measured, then the change in frequency can be correlated to the change in mass, and then to the change in film thickness and the incremental corrosion rate.

As expected, as the level of technology increases to allow continuous monitoring of corrosive gases, the cost of instrumentation increases.

Standards

Once the corrosion problem is recognized, there must be some method of determining if the measured rate of corrosion is harmful. If protective measures are taken, then it must be determined what rate of corrosion would be acceptable to the compressor or process. While these results often are compared to some standard, there are currently no standards specifying allowable levels of corrosion for the air compressor industry. As a result, compressed air system operators must define their own guidelines, or adopt a standard from another industry.

A widely used corrosion standard is the Instrument Society of America (ISA) standard ANSI/ISA-S71.04-1985: Environmental Conditions for Process Measurement and Control Systems: Airborne Contaminants. This standard is used to correlate the corrosion film thickness with electronic instrument reliability, and uses four levels to describe corrosion problems.

Level G1 describes a mild environment. Corrosion should not be a factor in determining equipment reliability. Corrosion film should build up on a copper sample at less than 300 angstroms (1 angstrom = 10-10 meters) when measured over a 30-day period.

Equipment in moderate or G2 environments shows measurable effects of corrosion and may have decreased reliability. Copper corrosion film buildup should be between 300 and 1000 angstroms over a 30-day period.

Level G3 is a harsh environment. There is a high probability that corrosive attack will occur and some effort should be made to protect the equipment. A corrosion film buildup on a copper strip of between 1000 and 2000 angstroms in a 30-day period indicates a G3 environment.

A corrosion film buildup of greater than 2000 angstroms over a 30-day period on a copper sample indicates a GX or severe environment. Only specially designed and packaged equipment can be expected to survive. Although this standard was written for electronic instruments, it can be used to classify the corrosive nature of the air stream in a compressed air system. If, for example, the environment was classified as G3 or GX, there should be some cause for concern.

Protecting Your System

Once the air has been identified as corrosive, something must be done to protect the internal mechanisms of the compressed air system. The most common method of removing chemicals from the air is gas-phase filtration or dry scrubbing the air. This compressor protection system removes gaseous contaminants from the air stream by way of adsorption, absorption, and chemisorption.

An adsorbent media, such as granular activated carbon, will attract other substances and hold them on the surface of the adsorbent particle. Absorption is a similar process in which the attracted chemical penetrates into the internal structure of the filtration media. The major problem with these two types of media is that they will sometimes release the captured gases.

Carbon based media such as coconut shell carbon will release lower molecular weight gases in favor of high molecular weight gases. A chemisorbent media, such as potassium permanganate-impregnated alumina, will chemically react with some contaminant gases, oxidizing them and removing the possibility that they could be released. A filtration system supplier can recommend the appropriate chemical media type.

The chemical media, which must be occasionally replaced, works in conjunction with a standard particulate filter system to protect the compressor from corrosive gases.

However, if the internal compressor parts have been corroded, qualified service personnel from your compressor manufacturer should decide whether the parts should be repaired, replaced, or returned to service. The best decision, therefore, is to assess the corrosive nature of the environment and select a protection system prior to the purchase of a new compressor, or the rebuild of an old one. If a severely corrosive environment exists, then a protection system should be used from the first startup of the compressor.

Sidebar:

While corrosive gases can be found everywhere, the most common source is the atmospheric discharge of some process near the compressed air system's operating location. The air pollution in a major city can also be damaging to an air compressor. Even levels of contaminants safe for humans can affect the reliability of a compressor.

As the pressure rises in a compressed air system, so does the temperature and density of the air. This density increase will cause moisture to condense out of the air. If the air is cooled after compression, more moisture will condense. When corrosive gases are present in the environment, the high temperature and moisture content inside the compressor will increase the rate of corrosive chemical reactions. (The moisture content of the air depends on the relative humidity of the air taken into the compressor.) High levels of relative humidity can result in more condensate forming in the compressor.

While many corrosive gases will dissolve into this water, some gases, such as chlorine, will form hydrochloric acid when dissolved in water. Sulfur dioxide will combine with condensed water to form sulfurous acid. If there is an electrical potential difference between two dissimilar metals, and a conductive solution of some substance dissolved in water exists between the two metals, then electrolytic or galvanic corrosion can occur. And in situations where lubrication is present, additional chemical reactions will occur and further compound the issue.

The following chart demonstrates some examples of corrosive gases, their sources, and the materials they affect. While all corrosive gases listed below are harmful to air compressor components, the materials most susceptible to corrosive attack are copper, aluminum, and cast steel. Cast steel is used for many centrifugal compressor components such as the casing and some diffusers. Aluminum is used for diffusers and copper is commonly used for intercoolers. Admiralty (70-73% copper, 0.75-1.2% tin, remainder zinc) or stainless steel are sometimes used in intercoolers to reduce the possibility of corrosion, but these metals are still susceptible to corrosive attack, though at a slower rate. The most susceptible components in a centrifugal air compressor are usually the coolers.

Corrosive Gas

Inorganic chloride silver, compounds (i.e. chlorine, chlorine dioxide, hydrogen chloride)

Source

chlorine manufacturing, paper mills, aluminum manufacturing, combustion and refuse decomposition

Metals Affected

copper, tin, and iron alloys

Even when concentrations are at low parts-per-billion levels, they will still readily attack these metals, but low humidity levels will help reduce the corrosion threat.

Corrosive Gas

Hydrogen fluoride

Source
aluminum manufacturing, steel manufacturing, ceramics manufacturing, electronic device manufacturing, fertilizer manufacturing, and fossil fuels.

Metals Affected

copper, tin, silver and iron alloys

Hydrogen fluoride reacts in the same manner as an inorganic chloride compound even though it is a member of the halogen family.

Corrosive Gas

Active sulfur compounds (i.e. hydrogen sulfide, elemental sulfur, and organic sulfur compounds)

Source

sulfur manufacturing, fossil fuel processing, ore smelting, foundries, wood pulping, sewage treatment, combustion, and geothermal emissions

Metals Affected

copper, silver, aluminum, and iron alloys

Active sulfur compounds will readily attack the above listed metals, even at low concentrations. The reaction between sulfur compounds and the affected metals does not require high levels of humidity, but when combined with water and small amounts of inorganic chlorine compounds, they become much more reactive. Levels of hydrogen sulfide that are safe for humans can cause serious damage to air compressors.

Corrosive Gas

Sulfur oxides

Source

sulfuric acid manufacturing, ore smelting, combustion, and tobacco smoke

Metals Affected

reactive metals

Sulfur oxides are released into the atmosphere during combustion of high sulfur content fossil fuels. Low levels of sulfur oxides (measured in parts-per-billion) can passivate reactive metals, slowing corrosion. These gases will attack metals when present at higher levels though, and in the presence of water, dissolve to form sulfurous and sulfuric acid. Levels of sulfur dioxide that also are safe for humans can cause serious damage to air compressors.

Corrosive Gas

Nitrogen oxides (NOX)

Source

chemical manufacturing,combustion, and automobile emissions

Metals Affected

most common metals

Oxides of nitrogen (NOX) are combustion by-products of fossil fuels. When dissolved in water, some oxides of nitrogen will form nitric acid, which will attack most common metals. These gases are also believed to be catalysts in the corrosion of base metals by chlorides and sulfides.

Corrosive Gas

Ammonia, amines, and ammonium ions

Source

fertilizer manufacturing, cleaning products, sewage, and microbes

Metals Affected

copper and copper alloys

Ozone, chlorine, and chlorine dioxide
chlorine manufacturing, aluminum manufacturing, paper mills, electronic filters, combustion, automobile emissions, and refuse decomposition many elastomers and plastics

Ozone, chlorine, and chlorine dioxide are powerful bleaching and oxidizing agents, and will attack the surface of many elastomers and plastics. Ozone also may be a catalyst in the corrosion of metals by sulfides and chlorides.

Reference :
Website http://air.irco.com

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