Stainless steel, is one of the most preferred material for pharmaceutical applications because of its corrosion resistance and excellent durability. ( Read Alloys used in the Manufacture of Pharmaceutical Equipment). But by "stain-less" we actually mean stain-resistant. One of the main issues faced by pharmaceutical engineers, is a phenomena that often affects high-grade stainless steel equipment - "Rouging" .
Rouging of stainless steel is the result of the formation of a layer of iron oxide, hydroxide or carbonate on the surface of the stainless steel. It is a thin layer with reddish-brown, yellow or black tones, and it can indicate a de-stabilization in the naturally occurring passive layer of stain-less steel. Some forms of rouging can wipe of easily while others can stick to surfaces and, if left untreated, can cause product contamination. The occurrence of rouge is typical in water treatment systems, and pharmaceutical processing equipment. It can occur on the inner surfaces of storage vessels, buffer vessels, distillation systems, pumps, inline filters and other process equipment.
In the pharmaceutical industry rouge can be classified as
Type I : An easily wipe-able form or rouge that does not stick to surfaces and does not affect the surface of the stainless steel passivated layer. It usually originates from an external source.
Type II : Rouging that sticks to surfaces and alters the Fe/Cr ratio of the stainless steel surface. The rust colored deposits can cause underlying damage
Type III : Rouge that vitrifies into the surface, that is usually formed in high temperature steam systems. Type III rouge is black in colour and a form of magnetite.
The exact cause of rouging is highly debated in the pharma industry, and is not fully understood. Predicting when it will occur is difficult, as in some cases it can appear in just a couple of months, and in others it can take years before it occurs. However, it is universally agreed that the occurrence is exacerbated by high temperatures and highly corrosive environments. Rouge occurs in most pharmaceutical and biopharmaceutical facilities and often is associated purified-water systems, clean- steam systems, buffer-preparation tanks, filling lines, vial washers and steam sterilizers, according to Phamatech. Some of the known causes of rouging include:
1. Exposure to highly aggressive environments, such as steam, high pressure, high temperatures and corrosive products, and chlorides
2. Poor quality of stainless steel , poor welding, and construction, and the degree of surface roughness in the equipment.
3. Vulnerabilities and defects in the passive layer caused by inadequate passivation.
4. The presence of iron contaminants in the material that comes in contact with the system.
5. Frequently seen in hot water processes above 60 C that cause iron atoms to increasingly diffuse to the surface and to react with the oxygen present at the surface to form oxides and hydroxides
6. Improper cleaning methods, aggressive cleaning agents, and in adequate cleaning procedures can be a source of contamination
1. A Source of Contamination: Although rouge itself is not considered a contaminant, it creates a rough surface which can allow biological and other residue to grow and spread to the product. Rouging can accelerate the formation of bio-films that can cause contamination.
2. Reduced Life of Equipment: Serious rouging can cause "pitting" (small pits and holes in the surface) or stress cracking. This kind of deep corrosion can lead to inefficiencies in the system.
3. Can cause Blockages and Operational Problems: A build up of rouge in pipes and filters can cause blockages and mechanical failure.
4. Increases Equipment down-time : Cleaning and de-rouging procedures are time consuming and can be expensive
1. Selecting the right material and surface finish for your equipment: Stainless steel composition is very important to avoid rouging. 316L is the most popular stainless steel for processing equipment as it has low carbon content. Ni and Mo content protects against chloride and are resistant to corrosion. Alloys such as C-22 offer the highest protection. Electropolishing process vessels, and piping equipment smoothens the surface and will reduce the occurrence of rouge.
2. Choosing the right equipment provider:When fabricating processing equipment, the right in-process controls for welding and installation, avoiding iron contamination, correct de-greasing and passivation procedures and familiarity with your product and ASME-BPE regulations is vital for long lasting equipment.
3. Regular inspection: Facilities should have procedures in place for periodically inspecting systems and process equipment as well as for cleaning, derouging, and passivating equipment.
4. Avoiding corrosive environmentsAvoid chlorides and sulfides, high temperature steam, and if possible, lower the operating temperature, and adjust ph of products that move through the system to reduce rouging
5. Correct cleaning proceduresSystems and equipment should have scheduled maintenance and cleaning, that efficiently removes residue
De-rouging is the process of removing the rouge from the surface of the equipment. Then, Passivation of stainless steel, a corrosion reaction is carried out under controlled conditions, to grow a very thin, layer of film on the surface that is protective against further corrosion.
Cleaning/De-Greasing - First, a thorough pre-cleaning must be done on the equipment. The surface is treated with an alkaline detergent that removes surface and organic residue. Any organic oily or fatty residue is also removed, known as de-greasing.
De-Rouging -
Passivation - Chemical methods are used to regenerate the passive layer and provide increased protection. The thin chromium oxide layer on the surface of the stainless steel should be developed to avoid future corrosion.
Disposal of the Cleaning Chemicals - Proper care should be taken to neutralize and dispose of the cleaning acid and alkaline chemicals used in this process.
Preventative treatment continues to be the best method, to avoid rouging, and save cost and down time in the long run. Choosing the right equipment and process environment, regular inspections, as well as scheduled CIP treatments throughout the life of the equipment are very effective in maintaining equipment. Routine cleaning to remove organic residues with an alkaline cleaner and low concentration acid rinses can be used periodically to avoid severe rouging that will be time-consuming and expensive to correct.
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