Technical Data Monograph. Stainless Steel Instrument Maintenance Using Prolystica Restore Descaler & Neutralizing Detergent

Transcription

1 Technical Data Monograph Stainless Steel Instrument Maintenance Using Prolystica Restore Descaler & Neutralizing Detergent

2 Table of Contents 1. Background Experimental Studies Passivation Conclusion References...8

3 1Background Maintenance of stainless steel instrumentation and equipment is an important factor when making instrument reprocessing choices. The wrong choices in destaining and descaling chemistry can result in further reduced functionality and instrument life for already damaged instruments. Prolystica Restore Descaler & Neutralizing Detergent was formulated to destain previously damaged stainless steel surfaces, while improving the passive layer of the stainless steel and enhancing resistance to future corrosion and damage. As a result, it extends the use life of instruments and positively affects the bottom line. What is stainless steel? Stainless steel is comprised mainly of iron and chromium. This combination of materials results in a surface that stains less, not one that is stain-proof. There are a variety of other materials that give stainless steel desired properties, such as carbon for hardness and molybdenum for protection against localized corrosion (i.e., pitting ). The varying classes or series of stainless based on these properties consist of the 200, 300, and 400 series. The 300 and 400 steel series are most widely used in healthcare applications. For example, 316L stainless steel has excellent general corrosion resistance at high temperatures and in the presence of water (steam), and is often used for manufacturing washers and disinfectors. The 420 stainless steel is a harder material, and is often used for surgical blades and scissors because it has good edge retention. What is a passive layer? The uppermost surface of stainless steel is called the passive layer, which can be considered the skin of a stainless steel instrument. This layer is the result of a controlled oxidation that seals the surface. The integrity of this skin derives primarily from the amount of chromium oxide in relation to the amount of iron oxide (the higher the chromium to iron [Cr/Fe] ratio, the higher the quality of the passive layer). Controlled treatment with certain chemicals can result in an enrichment of the amount of chromium at the surface. This treatment is referred to as passivation, and is shown in Figure 1. Figure 1. Formation of the Passive Layer 3

4 The goal of a passive layer is to make stainless steel surfaces less reactive, and therefore, less susceptible to corrosion. Maintaining or enhancing this layer can result in equipment and instruments that last longer. What types of corrosion are found in the healthcare environment? The variety of soils, wear and exposure to repeated extreme processing conditions or incompatible chemistries can result in corrosion and stains forming on the surface of stainless steel surgical instruments, even when properly passivated. Failure to remove these stains or corrosion can result in either chemical or microbial contamination. For example, rust that formed on the edge of a surgical blade can flake off during use. A pit on a hemostat can expose carbon in the lower layers of the stainless steel, serving as an accumulation site for bacteria. If the bacteria spreads deep into the pit, it can be difficult to clean / sterilize, and may even lead to additional corrosion if not treated. Heat and friction can exacerbate this phenomenon. The two most common types of corrosion are the formation of rouge ( rust ) and pitting. Rust can form over time, especially in moist environments and when chlorides (e.g., salt) are present. It can also occur if two different metals are in contact under certain conditions, which is referred to as galvanic corrosion. An example of this would be rust forming on an aluminum tray underneath a pair of stainless steel scissors. Rust generally occurs on a large surface of the stainless steel, and therefore, is usually easy to detect. In contrast, pitting occurs on a few small areas on a stainless steel surface, and can go undetected for a long period of time. Pitting is often initiated by chloride attack or weaknesses or imperfections at the surface. Since the metal composition below the surface is different than that at the passive layer, when the surface is compromised, galvanic corrosion can occur on the same piece of stainless steel, causing pitting to form and grow. Staining or scaling can occur if stainless steel is routinely treated with water or steam that contains a high amount of metal ions in solution (e.g. zinc, copper, calcium). Other causes of staining or scaling include overuse of acid rinse agents, and when chemicals used for boiler treatment carry over into the steam used to treat instruments. All of these phenomena can damage stainless steel instruments and equipment, and are indications of the breakdown of the passive (protective) layer. However, improper removal of these stains / corrosion products from those surfaces can result in further damage to the outer surfaces of stainless steel. How is Prolystica Restore Descaler & Neutralizing Detergent different? Prolystica Restore Descaler & Neutralizing Detergent can remove stains and rust from a stainless steel surface while restoring the protective passive layer, resulting in a surface that is more resistant to corrosion and pitting. In addition, it is a phosphate-free formula containing a blend of organic acids. One of the acids, citric acid, is recognized by the American Society for Testing and Materials (ASTM) as a passivating agent. In contrast, phosphoric acid, which is commonly used in acid descaling products, is not an ASTM approved passivating agent. Further, the use of phosphorous compounds (phosphates) in detergent systems has recently come under much scrutiny due to their negative impact on the environment, and many municipalities are now controlling the concentrations of phosphates in effluent. Since Prolystica Restore Descaler & Neutralizing Detergent does not contain phosphoric acid, these restrictions are not an issue when it is used. 2Experimental Studies Purpose Prolystica Restore Descaler & Neutralizing Detergent is a concentrated, liquid detergent designed for manual soaking of surgical instruments or use in automatic washers/disinfectors. The purpose of these studies was to demonstrate the key benefits of Prolystica Restore Descaler & Neutralizing Detergent on corrosion removal and pitting resistance. Methods, Results and Discussion To evaluate the effectiveness of Prolystica Restore Descaler & Neutralizing Detergent in derouging of instruments, stainless steel hemostats were used. To evaluate the passivation characteristics of Prolystica Restore Descaler & Neutralizing Detergent 316L stainless steel coupons were employed. 4

5 Derouging Rouge (rust) was generated on the surface of stainless steel hemostats by soaking in a solution of sodium chloride (salt solution) in the presence of carbon steel. Each hemostat was then soaked in a heated dilution of Prolystica Restore Descaler & Neutralizing Detergent for 30 minutes. The hemostat was rinsed with deionized water. If any spots were visible, they were brushed with a tooth brush and then the hemostat was rinsed again with deionized water. Post-treatment photographs of each hemostat were taken. Figure 2 compares a damaged, untreated hemostat to one treated with Prolystica Restore Descaler & Neutralizing Detergent. The staining/rouge is clearly visible on the untreated hemostat, whereas the hemostat that was treated with Prolystica Restore Descaler & Neutralizing Detergent looks brand new. Figure 2. Derouging ability of Prolystica Restore Descaler & Neutralizing Detergent on hemostats. 3Passivation As previously described, passivation is a controlled treatment of stainless steel to enhance protection of the surface against corrosion. Different instrumental techniques were used to evaluate how well Prolystica Restore Descaler & Neutralizing Detergent passivates and protects stainless steel materials. When evaluating the performance of Prolystica Restore Descaler & Neutralizing Detergent there are three key questions to be answered. 1. Does passivation take place? Potentiodynamic polarization was used to assure that the passivation process took place Does the surface of the metal change as a result of the passivation process? X-ray photoelectron spectroscopy (XPS) was used to evaluate the molecular composition at the stainless steel surface. This provides an assurance that the protective metal species were formed during the process of passivation.

6 3. Does the passive layer perform its intended function and enhance resistance to corrosion? Critical pitting temperature (CPT) analysis was used to predict how well the protective passive layer resisted the formation of pitting corrosion. Potentiodynamic polarization: This electrochemical technique was used to evaluate the passivation behavior of Prolystica Restore Descaler & Neutralizing Detergent on stainless steel and demonstrates that the passivation process took place. In this test, a stainless steel electrode was attached to an electrochemical apparatus and immersed in a use-dilution of Prolystica Restore Descaler & Neutralizing Detergent. A graphic representation of the relationship of current and potential (voltage) is pictured. The difference between the corrosion potential (E corr ) and the pitting potential (E pit ) is the passive region and an indication of the quality of the passive layer on the stainless steel surface. The potentiodynamic polarization scan for Prolystica Restore Descaler & Neutralizing Detergent (Figure 3) has a passive region that is easily identifiable. Treatment with deionized water alone (Figure 4) results in no passive region, an indication that passivation did not take place. Figure 3. Potentiodynamic Scan of Prolystica Restore Descaler & Neutralizing Detergent Figure 4. Potentiodynamic Scan of Deionized (DI) Water 6

7 X-ray photoelectron spectroscopy (XPS): This technique provides an elemental breakdown of the stainless steel at the surface to verify an increase in more protective species (chromium oxide) during the passivation process. A surface Cr/Fe ratio of greater than 1.5 or greater indicates a chromium-rich surface, which is characteristic of a high-quality passive layer. Stainless steel coupons were treated under specified conditions of dilution, temperature, and time and then analyzed by XPS. A depth profile of the Cr/Fe ratio was determined for each treated stainless steel surface. Results from this study indicate that stainless steel passivated with Prolystica Restore Descaler & Neutralizing Detergent has a Cr/Fe ratio greater than 1.5 at the surface and has a substantial increase over that of an untreated surface (Figure 5). This improved level of passivation continues to about 15 angstroms deep into the stainless steel surface. These results confirm that passivation with Prolystica Restore Descaler & Neutralizing Detergent provides a substantial protective layer on the stainless steel surface. 7 Figure 5. XPS Evaluation of the Effect of Passivation of 316L Stainless Steel Using Prolystica Restore Descaler & Neutralizing Detergent Critical pitting temperature (CPT): This analysis is used to determine the potential for pitting corrosion at or below the surface of stainless steel. The standard method utilized in this evaluation was ASTM G-150 (Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels). Voltage was applied to 316L stainless steel coupons while immersed in a sodium chloride solution. The temperature was increased 1 C (1.8 F) per minute until localized corrosion (pitting) occured. Pitting was indicated by a sharp rise in current and visible pitting confirmed post-exposure. A higher critical pitting temperature indicates a greater resistance to pitting corrosion. A summary of critical pitting temperature results can be found in Table 1. Stainless steel purposefully damaged by treatment with hydrochloric acid (HCl) (to damage any existing passive layer) had a pitting temperature of less than 10 C (50 F). Treatment of the damaged stainless steel with a phosphoric-acid based detergent resulted in a critical pitting temperature of less than 25 C (77 F) (below typical detergent wash cycle temperatures). However, treatment of a damaged stainless steel surface with Prolystica Restore Descaler & Neutralizing Detergent resulted in a critical pitting temperature of 71 C (160 F). These results demonstrate that using Prolystica Restore Descaler & Neutralizing Detergent will result in superior protection of stainless steel surfaces from pitting corrosion. Table 1. Summary of CPT Results Sample Description Critical Pitting Temperature Stainless Steel Damaged by HCl << 10 C Stainless Steel Damaged by HCl then passivated with 20% Prolystica Restore Descaler 71 C & Neutralizing Detergent Stainless Steel Damaged by HCl then passivated with 15% of a Phosphoric Acid-based < 25 C Cleaning Product 316L SS (untreated) 19.5 C (per ASTM G150 literature)

8 4Conclusion Prolystica Restore Descaler & Neutralizing Detergent is effective in removing corrosion products from stainless steel surfaces. Prolystica Restore Descaler & Neutralizing Detergent is also effective in passivating stainless steel surfaces, resulting in increased resistance to corrosion. Testing demonstrated a significant passive region when tested using potentiodynamic polarization techniques, an increase in the Cr/Fe ratio utilizing XPS, and an increase in CPT. 5References 1. ASTM A Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts 2. ASTM G (2004) Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels 3. STERIS Research and Development Laboratory Notebooks 6490: 14, 51-53; 6264: 49-50; 6371: Report CTL REF # R from Corrosion Testing Laboratories, Inc., September 7, 2009, Critical Pitting Temperature Determination of 316L SS After Various Passivation Treatments 5. Princeton Applied Research, Basics of Corrosion Measurements and Electrochemistry and Corrosion Overview and Techniques Document #M3481EN , Rev. A Printed 9/2011, STERIS Corporation. All rights reserved. Printed in USA. STERIS Corporation 5960 Heisley Road Mentor, OH n USA n