Spotting, Staining, and Corrosion of Surgical Instruments

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Spotting, Staining, and Corrosion of Surgical Instruments

Herbert J. Kaiser, PhD; Patrick Schwab, MBA; Jason F. Tirey, MA

Figure 1: Formation of passive layer. Gray = metallic iron; Green = metallic chromium; Red = oxidized iron; Orange = oxidized chromium.

Surgical instrument spotting, staining, and corrosion are serious problems in many healthcare facilities. These problems can be avoided provided careful attention is given to the method of processing the instruments and possible causes are understood. This article covers various reasons to monitor the instrument-processing scheme along with understanding basic instrument construction itself. Symptoms of problems are presented along with descriptions of common sources of instrument problems.

Spotting, staining, and corrosion of surgical instruments can impair their function. For example, a hemostat may not open because of corrosion in the box area, scissors and scalpels could become dull, and instruments could break during surgery as a result of severe corrosion. Spotting, staining, and corrosion also interfere with sterilization. Spores can be protected from destruction by the layers of iron oxide (rust). Corrosion can result in a shortened instrument life, which results in increased cost.

There are thousands of different types of surgical instruments available on the market. The vast majority of these instruments are made of stainless steel. Some of these instruments contain hardened inserts that are typically identified by their gold handles. The composition varies according to intended use. For example, a cutting edge would be made of softer steel than an instrument used for grasping. The harder stainless steels are typically found in the 300 series stainless steels. The softer stainless steels, such as those used for surgical blades, are found in the 400 series stainless steels. The members of the 300 series are made of more corrosion-resistant materials.

Stainless Steel

What's in stainless steel? Stainless steel is composed of iron, carbon, chromium, nickel, manganese, silica, and many other metals in smaller quantities. The amount of each of these components depends upon the grade of stainless steel. Generally, the higher the chromium content, the more corrosion resistant the metal is. It is important to remember that stainless steel means stainless steel, not stain-proof steel. All types of stainless steel will eventually become corroded and stained. Also, while a surface of an instrument may look bright and shiny to the naked eye, under microscopic examination the surface is actually very rough. The rough surface allows for entrapment of impurities from soils and water.

Figure 2: Simple diagrams comparing spotting, staining, and corrosion on an instrument's surface. Spotting lies loosely on the surface. Staining is integral with the surface. Corrosion penetrates the surface.

One of the important qualities of stainless steel that makes it more corrosion-resistant than carbon steel (the steel that is found in cars) is its ability to become passivated (Figure 1). According to Webster's Ninth New Collegiate Dictionary, passivation means "to make inactive or less reactive." In the case of stainless steel used in surgical instruments, the description "less reactive" is more appropriate. Stainless steel is indeed reactive, meaning that it will corrode and become stained under certain conditions. This staining or corroding occurs much less frequently than in carbon steel. Carbon steel does not contain the additional agents that allow passivation to occur.

The difference in reactivity between stainless steel and carbon steel is a layer of certain metals present at the surface of the stainless steel. This layer is called the passive layer. The passive layer is composed primarily of chromium and iron oxides. The amount of chromium and iron oxide varies depending on the type of stainless steel (e.g., 410, 416, 316, etc.). Layers with more chromium are generally more passive; that is, more resistant to corrosion. Carbon steel does not have this passive layer.

Passive Layers

Let's examine how a passive layer is formed on stainless steel (Figure 1). When the parts of the surgical instrument are first made they do not have a thick passive layer. When these parts are exposed to air, the chromium and iron present in the stainless steel are oxidized. This forms a slightly passive layer at the surface. A more passive layer is then formed by treating the parts with chemicals that remove some of the iron from the surface but leave the chromium behind. This is called chromium enrichment of the surface. Chromium is the principal metal responsible for the passive behavior of stainless steel. If a thicker, more protective layer is desired, the parts are treated further with chemicals that cause the layer to become thicker. The surgical instrument is then ready to be assembled if required.

Stainless steel has the unique ability to heal itself. A good analogy is the human skin. If the skin is broken from a cut, for example, as long as the cut is cleaned and exposed to the air it will heal itself. The same is true with a cut through the passive layer of stainless steel. If the original passive layer is disrupted, simple exposure to air will form another passive layer where the disruption occurred. This new passive layer may not be as thick as the original layer, but it will still provide some protection. However, the disrupted passive layer must have time to heal itself in the presence of air. If the scratch is covered by soil, it is not exposed to air, and therefore cannot heal itself. Also, if the scratched area is exposed to harsh chemicals, the area will begin to corrode. Some chemicals actually attack the passive layer itself. This either thins the layer, making it more vulnerable to scratches, or the chemicals continue to attack the metal beneath the passive layer, which causes corrosion.

There are many possible causes of damage to the passive layer. Physical damage, for example scratches, is a primary cause of damage. Scratches to the passive layer can occur either through improper handling of the instrument or through normal usage. Harsh cleaners can also cause damage to passive layers. These harsh cleaners include both highly acidic and highly alkaline cleaners. Many times destainers, which are highly acidic, are overused. While destainers are necessary in some cases to remove some areas of corrosion and tough stains, they should not be used on a daily basis, as this will eventually erode the passive layer, making it thinner and providing less protection. Soil residues that are allowed to dry on the surface of instruments can cause damage to the passive layer. Soils should not be allowed to dry onto the surface of instruments. The misuse of disinfectants can also cause damage. This can occur when instruments are allowed to soak overnight in strong disinfectant solutions. It is important to follow the label instructions on disinfectants. This includes concentrations and exposure times. If a disinfectant is recommended for use at 1 ounce per gallon, it should not be assumed that it would be twice as effective at 2 ounces per gallon. This increase in concentration can cause damage to the surgical instruments. Exposure to chlorine compounds like bleach is very detrimental to the stainless steel surface. Stainless steel should never be exposed to chlorine for extended periods nor soaked in sodium chloride solutions. A common practice in some operating rooms is to immerse stainless steel instruments in saline solutions. Exposure of instruments to either chlorine or sodium chloride is one of the most harmful things that can be done to stainless steel.

An often overlooked source of staining or damage to the passive layer is residues from reusable instrument wraps. It is important to understand the process by which these reusable wraps are cleaned in order to be aware of potential sources of damage to surgical instruments. In home laundry a detergent is simply added during a wash cycle and the washing machine processes or cleans the clothing or material under set conditions. In a commercial laundry, where instrument wraps are normally reprocessed, a totally different process occurs. Typically, a very strong alkaline detergent is used to clean the wraps. This is followed by a rinse to remove the gross detergent. An acidic rinse, called a laundry sour, is then used. This laundry sour neutralizes any alkalinity remaining in the wraps from the detergent. This step is followed by further rinsing. If the laundry sour does not neutralize the alkalinity in the wraps an alkaline residue will remain. This alkaline residue can then be reactivated under the high-temperature and humidity conditions of a sterilizer and leach onto the instruments, causing them to become stained or corroded. The same is true if the alkalinity is over-neutralized and an acidic residue is present. Also, if there is overall improper rinsing of the wraps, surfactant residues can be left in the linens.

If the wraps are suspected of causing staining of the instruments they can easily be checked for laundry residues. There is a simple test to determine if the wraps are alkaline or acidic in nature. This involves boiling deionized water and determining its initial pH. A sample of the wrap is then immersed in the boiling water and boiled for another set time period. After this time period the pH is checked again. A difference between the initial and final pH of more than 0.5 pH units indicates there is residue present. If the final pH is 0.5 units greater than the initial pH, the residue is alkaline in nature; if the pH is 0.5 units less than the initial pH, the residue is acidic in nature.

Additionally, if the water is foamy after boiling the wrap, there may be detergent residues present. If the linens are found to be acidic, alkaline, or if there are detergent residues present, the laundry should be consulted to make sure proper procedures are being taken to avoid and eliminate these problems.

Another cause of damage to passive layers is hard-water deposits.1 These hard-water deposits can form on instruments either from using detergents that are not hard-water tolerant or from rinsing instruments with water other than deionized or softened water. If instruments are rinsed with water other than deionized or softened water, deposits will form on the instruments that can entrap other harmful water chemicals. These deposits can act as seed points for corrosion to occur.

Flash sterilization can also eventually cause damage to passive layers. This occurs primarily because of the rapid temperature change that the surface experiences during flash sterilization. While flash sterilization has become a somewhat normal procedure in many healthcare facilities, it should only be used in cases where other forms of sterilization are not available or applicable.

Spotting, Staining, and Corrosion

What is meant by spotting, staining, and corrosion? Spots are loose or semi-adherent deposits on the surface of instruments (Figure 2). These typically may be wiped off with a cloth with minimal friction. Instruments generally are not physically or chemically affected by spots if they are removed. Water mineral deposits, chemical residues, steam residues, and poor soil removal can cause spots.

Water mineral spotting can be avoided through the use of deionized water or softened water in the final rinse. The use of high-quality cleaning chemicals can avoid mineral deposit formation during the wash cycle. However, cleaning chemicals can cause spots themselves if they are not hard-water tolerant. The use of cleaning chemicals outside the recommended dilution rates may also cause spotting. Equipment should be maintained so that the appropriate amounts of cleaning chemicals are fed into the washer.

Spotting as a result of steam impurities can be avoided by making sure that the steam quality is kept high. This can be done by making sure that the steam and boiler water are monitored on a regular basis. Inadequate maintenance of the boiler can cause boiler chemicals to be carried over into the steam system. This will cause spotting to occur on instruments. It is also important to clean any filters and traps that are associated with steam sterilizers. Additionally, steam lines should be flushed after any major adjustments to the boiler.

Spots resulting from poor soil removal can be avoided through the use of high-quality cleaning chemicals. Again, it is important to use proper dilution rates and not dilution rates that exceed those recommended by the manufacturer. The use of enzyme presoaks is recommended for initial protein soil removal.

In contrast to spots, stains are tightly adherent deposits on the surface of instruments. These stains can be an integral part of the surface. However, as with spots, the instruments generally are not physically affected by the presence of the stains. Stains can be caused by the replating of metals, impure steam, and many other causes.

The replating of metals is a phenomenon called galvanic corrosion. This is where metals that are dissimilar are exposed to a solution at the same time. One metal corrodes in the solution and deposits onto the other piece of metal. In essence, a small battery is established. A symptom of the replating of metals is a rainbow-like appearance on the surface of an instrument. In addition, the stain can appear as a gold or rusty tint on instruments. This can be avoided by segregating dissimilar metals and washing them at different times. However, the segregation of dissimilar metals is not always feasible. When this is the case, a cleaner that is safe on soft metals (e.g., brass, aluminum, and copper) can be used to minimize the effect. Normally, these cleaners are neutral pH cleaners.

To avoid staining caused by disinfecting solutions, the soak time should be limited (i.e., follow label directions closely). Increased concentrations of disinfectants should not be used. In addition, iodophors (iodine-containing products) should not be used on stainless steel instruments.

As with spots caused by steam impurities, stains from steam can be avoided by keeping the steam quality high. A common stain seen after sterilization is a rainbow or bluish appearance on instruments. This typically is caused by the presence of an excessive amount of neutralizing amines in the steam. It should be noted that the presence of neutralizing amines is normal in steam. They are used to prevent corrosion of the steam lines. However, an excessive amount of neutralizing amines can be prevented by working closely with the boiler operators.

There are two common stains associated with low-impingement washers. Low-impingement washers are those that utilize a highly alkaline detergent followed by an acidic rinse to neutralize the alkalinity. The stains encountered in these washers are typically either black or rust in color. The black color is a result of the alkalinity of the detergent not being sufficiently neutralized. The rust color is a result of over-neutralization of the alkalinity. This usually appears after the sterilization process. To avoid discoloration of the instruments in these systems, it is important to ensure that alkalinity is being properly neutralized. This can be done by determining the pH of the rinse water before it is used and then rechecking its pH after rinsing. If the pH of the rinse is 0.5 pH units higher than it initially was, the alkalinity is not being neutralized properly. The amount of neutralizer being fed should then be increased. If the pH of the rinse is 0.5 pH units lower than it initially was, the alkalinity is being over neutralized. The amount of neutralizer being fed should then be decreased. If the pH of the rinse is within 0.5 pH units of its initial value, the alkalinity is being properly neutralized. 

Herbert J. Kaiser, PHD, is a principal scientist at STERIS Corporation. He has 17 years of experience in cleaning technologies and is the sole inventor listed in five US patents for various industrial treatment schemes.

Jason F. Tirey, MA, is a scientist at STERIS Corporation with two years of experience in cleaning and surface technologies.

Patrick Schwab, MBA, is a product director, marketing, at STERIS Corporation with 14 years of experience in healthcare.

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