Bleach is known to be corrosive to metals that are commonly found in pharmaceutical, bioprocessing, and medical device work environments. Two commonly used types of stainless steel coupons were exposed to household bleach and sodium dichloroisocyanurate (NaDCC) solutions over a period of eight weeks. The rate and degree of corrosion exhibited by the stainless steel coupons were compared. Bleach diluted at 1:10 and 1:50 showed corrosion of the stainless steel. Coupons exposed to NaDCC solutions at levels of 187 and 937 ppm active chlorine did not show corrosion, suggesting that NaDCC can serve as an effective alternative disinfectant to liquid bleach.

Introduction

Liquid bleach (sodium hypochlorite solution) is used as a disinfectant in pharmaceutical, bioprocessing, and medical device facilities commonly at a 1:10 dilution (one part bleach combined with nine parts water). However, bleach is known to be corrosive to metals and can cause damage to some plastics. Even with these drawbacks, bleach is commonly used because it kills a large spectrum of microbes, is widely available, and is easy to use. Sodium dichloroisocyanurate (NaDCC) is a bleach alternative available in a solid tablet form. It also kills a large spectrum of microbes, is widely available and is easy to use.

Objective

The objective of this study is to compare the corrosion caused by bleach and NaDCC solutions at various concentrations on two common types of stainless steel surfaces and to demonstrate the advantages of using NaDCC over bleach as part of a disinfection program.

Three coupons were submerged in 400-mL beakers containing 250 milliliters of one solution listed above ensuring complete coverage of the coupons. The beakers were then covered with plastic wrap. The coupons were removed from the beakers daily and wiped dry for visual inspection. Any differences were documented (Table 2) and photographed, as shown in Figures 1 - 17. Noted differences include changes in color, rust and corrosion, pitting, gas evolution while the coupons were submerged in solution, and metal deposition on the glass surface of the beaker.

After inspection, fresh solutions were prepared for each coupon-solution combination, and the coupons were placed back in their original beakers. The beakers were maintained under an ISO Class 7 environment.

The study was conducted over eight weeks. However, the undiluted bleach samples were maintained an additional four weeks until metal was deposited on the glass beakers.

Results

A description of the progression of the coupons corroding in the test solutions follows. After four days, the 304L coupon in the 1:10 household bleach solution exhibited definite corrosion, pitting and staining as shown in Figures 1 - 2. The 316 coupon in the 1:10 household bleach solution also exhibited corrosion, but to a lesser extent.

By eleven days, the glass beaker sides containing the 304L coupon in the 1:10 bleach solution were contaminated with rust. The 304L and 316 coupons were clean in the water, 200 ppm and 1,000 ppm NaDCC solutions.

At two weeks

There was pitting, corrosion and staining of both 304L (Figures 3 – 4) and 316 1:10 bleach solutions and full-strength bleach solutions. Notable pitting is shown in the photograph above on the edge of a 304L coupon exposed to the 1:10 bleach solution. The corrosion spread across the coupon surface due to the rust falling down the coupon face (Figure 3).

At three weeks

In Figure 5, the photograph shows rust floating in the beaker containing 304L coupons exposed to the 1:10 household bleach solution. The photograph shows corrosion starting near the center of the face of a 304L coupon when exposed to the 1:50 household bleach solution in Figure 6. In Figure 7, the photograph above shows pitting beginning on the edge of a304L coupon exposed to undiluted household bleach. The photograph shows the clean face of a 316 coupon exposed to the 1,000-ppm NaDCC solution in Figure 8. In Figure 9, the photograph shows rust dispersed in the undiluted bleach solution containing the 304L coupons.

By four weeks, the 304L coupons in undiluted bleach were evolving a gas as evidenced by bubbles rising in the solution from the coupon edges and faces, and, between observations (one time), rust was splattered on the plastic wrap beaker cover.

At 5 through 7 weeks, metal deposition began on the glass beaker wall for the 1:10 bleach solutions containing the 316 coupons as shown in Figure 10 and in undiluted bleach solution of 304L coupons in Figure 11. The photograph in Figure 12 shows a 304L coupon exposed to an undiluted bleach solution. A hole has formed in the upper left corner of the coupon where the background is showing through, the entire coupon is discolored, and is also corroded and pitted.

At three months, rust was found at the bottom of the beaker that contained 304L coupons exposed to undiluted bleach in Figure 13. The photograph in Figure 14 shows rust remaining at the bottom of the beaker that contained the more corrosion resistant 316 coupons exposed to the more commonly used 1:10 bleach solution. Even though the experiment finished at eight weeks, 316 coupons were soaked for an additional month until metal deposits were formed.

At eight weeks, gas was evolving from the 316 coupons in addition to the 304L coupons in the undiluted bleach solutions.

The daily observations summarized by week are compiled in Table 2.

Key: C = Corrosion, the beginning of observation of corrosion; CC = Color change, the silver metal changes to a slight orange or darker color; G = Gas evolution, bubbles of gas evolving from the metal are visible in the solution; M = Metal deposition, dark and also shiny deposition on the beaker wall; P = Pitting, divots in the metal surface; and, R = Rust, a red, flaky material on the coupon face or edge or a deposit on the bottom of the beaker.

Discussion

Stainless Steel
There are different types of stainless steel. The addition of differing levels of other elements, molybdenum, nickel, manganese and chromium, to iron gives steel various properties, in this case, corrosion resistance.

The 304L and 316 stainless steel grades used in this experiment are designed to be more corrosion resistant, but not corrosion-proof. The difference in the metals content gives 316-grade stainless steel its higher corrosion resistance when compared to the 304L-grade.

Many pieces of equipment found in pharmaceutical, bioprocessing, and medical device facilities are constructed of 304L and 316 grades of stainless steel. This equipment is easy to clean and is resistant to corrosion by common chemicals and cleaners. However, continual use of bleach solutions as part of a disinfection program promotes corrosion of the equipment requiring the equipment to be replaced periodically.

Active Ingredient
The sodium hypochlorite found in bleach and NaDCC are active antimicrobial ingredients belonging to the oxidizing group of disinfectants. They form hypochlorous acid (HOCl) in water. The outcome of the interaction of HOCl with biomolecules found in microbes is cell death. (References 1 – 20)

Household bleach
Bleach is composed of sodium hypochlorite (NaOCl), sodium hydroxide (NaOH), and sodium chloride (NaCl). It is produced by passing chlorine gas through a dilute sodium hydroxide solution or by electrolysis of salt water. Sodium hypochlorite is reactive and may chlorinate organic compounds. (Reference 21) This solution contains HOCl in equilibrium with the hypochlorite ion as shown by the equations below.

NaOCl → Na+ + OCl-
OCl- + H+ ⇄ HOCl (hypochlorous acid)

Sodium dichloroisocyanurateNaDCC is formed through the pyrolysis of urea which forms cyanuric acid. The cyanuric acid is reacted with chlorine and sodium hydroxide to form NaDCC. (Reference 22) In water, the NaDCC forms HOCl in equilibrium with a complex mixture of various chlorinated cyanurate chemical species. (Reference 23)

NaCl2(NCO)3 + 2H2O ⇄ 2HOCl + NaH2(NCO)3

The NaDCC used in this experiment was supplied by Brulin & Company, Inc. (EPA Registration Number 71847-2-106) (Reference 24). On the label, the use instructions indicate adding one tablet per gallon for 1,000 ppm active chlorine.

Product use
The product use directions are similar for both products. To make a 1:10 bleach solution (5,000-ppm active chlorine), 1¾ cup bleach is added to one gallon of water with mixing as described by the bleach EPA label. For the 1,000-ppm NaDCC solution, one tablet is added to one gallon of water and allowed to dissolve through the effervescence, a process that takes less than two minutes. Each product is to be used per EPA registration label guidelines on hard, nonporous, inanimate surfaces that have been pre-cleaned. The products are applied to the pre-cleaned surface. The surface must remain wet for the prescribed contact time of ten minutes. If the surface dries before ten minutes, the reapplication of more solution is required. After ten minutes, the product may be allowed to dry on the surface or be removed.

While bleach is used to disinfect hard, nonporous, inanimate surfaces, as found on the Clorox® Bleach label (Reference 25), it also includes the following instructions, not found in the NaDCC product label:
• “Do not use this product on steel, aluminum, silver or chipped enamel.
• If used on metal, a solution of this product should be allowed to stand for no more than 5 minutes, and then rinsed off thoroughly with clean water; otherwise, it may slightly discolor and eventually corrode the metal.”

It is interesting to note that the second instruction for limiting exposure to five minutes is less than the prescribed contact time.

Conclusion

This study shows the corrosive effects of bleach solutions on 304L and 316 coupons at use levels typically employed for equipment disinfection in the pharmaceutical, bioprocessing, and medical device industries as part of their standard operating procedures. Figure 15 highlights the impact of bleach used on 304L coupons. The 304L coupons are discolored, corroded and pitted in contrast with the untarnished 304L coupons that were immersed in water for the experiment. The photograph in Figure 16 highlights the effects (discoloration, corrosion and pitting) on stainless steel coupons of bleach at common use concentration (1:10) compared to the effect (none) of NaDCC solution at its use concentration. The photograph in Figure 17 compares the effect of bleach and NaDCC at comparable active chlorine concentrations. The stainless steel coupons are still discolored and corroded while coupons immersed in NaDCC solution showed no effect.

Users of diluted bleach solutions on stainless steel must replace carts, hoods, biological safety cabinets, filter dryers and other equipment periodically due to corrosion following repeated exposure to bleach disinfectant solutions. Substituting NaDCC for bleach can reduce the frequency of replacement of expensive equipment incurring a cost savings while maintaining the disinfection level required for product manufacturing. This suggests using a NaDCC solution in place of bleach as part of a disinfectant rotation.

Jay C. Postlewaite, Ph.D., is the Senior Technical Advisor at Texwipe, an ITW Company. He has held technical positions in global regulatory service, research, manufacturing, and product development. His current research is focused in the cleanroom consumables and contamination control markets.

Wendy Hollands, MBA, is Business Development Manager at Texwipe, an ITW Company. She has managed multiple cleanroom consumable product lines and specializes in sterile product lines including wipers, pre-wetted wipers, swabs, and sterile alcohol.