Stainless steel is the primary construction material for pharmaceutical and biopharmaceutical processing equipment. Prized for its ability to resist corrosion, stainless steel comes in hundreds of types and subtypes. Austenitic stainless steel is the most common type in these applications. It is nonhardened, nonmagnetic, and easily welded, and it resists heat and chemical sterilization and corrosion. The most common austenitic stainless steels are 304, 304L, 316, and 316L. “L” refers to low carbon content (i.e., 0.03% instead of 0.08%). The base metal of 304 and 316 stainless steels is iron (62–65%), which is combined with chromium (16–20%), nickel (8–14%), and other components such as carbon, silicon, and molybdenum.
Rouge is a corrosion product of stainless steel that ranges in color from reddish brown to blue or black. The general types of corrosion are galvanic, pitting, stress, intergranular, crevice, and microbe-induced. Rouge can be classified into the following three types
[b]Type I: Oxidized metal particles, generated by an external source through erosion or corrosion, that are deposited on downstream surfaces. This rouge generally is easy to wipe off, leaving the underlying stainless-steel surface intact.
Type II: Rougecaused by in situ oxidation, which generally adheres strongly and sometimes indicates underlying surface damage or pits.
Type III: Black-oxide rouge generated by high temperatures or steam. The top layers of this rouge wipe off, but full removal often requires specific chemical treatment.[/b]
Rouge occurs in most Pharmaceutical & biopharmaceutical facilities and often is associated purified-water systems, clean-steam systems, buffer-preparation tanks (especially sodium-chloride or glycine-solution preparations), filling lines, vial washers (which typically are cleaned with hot purified water), and steam sterilizers. Rouge results from exposure to highly aggressive environments such as purified-water or -steam systems, chloride or corrosive products, and high-temperature stress or erosion. Other factors that cause rouge are nonstainless-steel components or inserts, improper welding, surface defects, and inadequate cleaning or passivation.
Facilities should have procedures in place for cleaning, derouging, and passivating new equipment, as well as for periodically inspecting in-use water systems and process equipment. New equipment often arrives with particulates and oily residues from buffing, polishing, and lubricating compounds. These residues must be removed, and the chromium-enriched passive layer restored.
The general procedure for derouging and passivation of 304 and 316 stainless-steel surfaces.
-Determine the type of rouge that is present. A laboratory evaluation is recommended.
-Preclean the surface with formulated alkaline chemistry as appropriate.
-Charge the system with a 10% concentration of formulated phosphoric or citric-acid-based detergent at 80 °C.
-Measure the initial iron concentration in the solution.
-Repeat the iron measurement at set intervals until a plateau is reached.
-Add formulated phosphoric or citric-acid-based detergent to a level of at least 15%.
-After a set interval, measure the iron concentration again. If the iron concentration did not increase significantly, continue agitation for three hours at 80 °C.
Repeat steps until the surface appears visually clean or until no additional iron is removed from the surface.
Drain the equipment, rinse it thoroughly with high-quality water, and blow it dry with clean compressed air or allow it to air dry.
Qualified engineering or maintenance staff should inspect in-use equipment for rouge. Surface inspection will help determine whether the equipment needs to be derouged and passivated. If no program is in place, a semiannual inspection is recommended as a starting point.