Introduction:- Pharmaceutical equipment and high purity water systems are designed so that product contact surfaces are not reactive, additive, or absorptive so the drug product is not adversely altered. High purity water systems are primarily composed of austenitic stainless steel (SS) materials due to their corrosion resistant and contaminant free properties. Passivation is performed to maximize the metal’s corrosion resistance. The stainless steel is sulfuric/nitric/hydroflouric acid pickled at the mill to remove manganese sulfide inclusions, scale, and other impurities or imperfections from the surface of the steel. As the steel is removed from the pickling bath, a thin oxide layer forms immediately over the surface. This oxide layer is what renders the stainless steel passive and non-reactive to corrosion. Any 300 series chromium steels containing 17% or more chromium that has been handled, welded, or worked should be passivated prior to service and suitably cleaned prior to passivation. Passivation is the method used to fortify the steel surface by strong oxidizing chemicals such as nitric acid.
The acid depletes the steel surface of acid soluble species, leaving the highly reactive chromium on the surface in a compounded oxide form.
Advantages of Passivation:- When SS systems are fabricated, the welding process destroys the existing passive film and compromises the metal’s ability to ward off the corrosive process. This is particularly applicable in those zones that are either heat affected or have had residues remain in contact with the metal surface for prolonged periods.
Passivating then provides a method to restore the integrity of the metals corrosion resistant surfaces that were disturbed. Passivation must be preceded by a cleaning process.
The Chemical Process
- Excessive electron depletion of the upper film and an inadequate supply of oxygen (molecular O2) will ensure the formation of surface corrosion products. When this occurs, the chromium (Cr+) separates from the surface and opens the way for oxidation of the iron (Fe) and nickel (Ni), lower in the metal lattice.
- Establishing a passive surface or film on austenitic SS is essential to maximize the corrosion resistance that the metals offer. Passive surfaces on these metals occur naturally when exposed to an oxidizing environment.
- Sources of oxygen include air, aerated water, and other oxidizing atmospheres. Formation of a substantial uniform oxidized corrosion resistant surface or film is the result of passivation.
- Besides natural occurring passivation, chemical and electro-chemical processes can be used to obtain an anodic oxide film. Nitric acid solution (HNO3), is an oxidizing acid (depletes electron from the metal surface) which erodes the metal. This initial reaction or oxidation resists further chemical reaction on the metal surface.
- Metals that have such a state are called “passive” and the phenomenon itself is called “passivity.”
- The chromium oxide film thickness typically ranges from 0.5-5.0 nm, averaging 2.0-3.0 nm. The chrome to iron ratio measured in atomic percent within the chromium oxide should be at least one with ratios of two or more being optimal.
Passivation Procedures:- Numerous procedures are available for passivating they share the commonality of consisting of four main steps which are:
- Wash (Solvent Degreasing)
- Water Rinse
- Acid Wash (Passivation Step)
- Final Water Rinse
Proper preparation of the metal surface to obtain a uniform non-defective passive film mandates the metal surface be completely clean and void of any organic or inorganic soils, free iron, metallic contaminants, or corrosive products.
- The First Step (Degreasing) of the procedure is designed to remove dirt, dust, oil, and grease. A water soluble detergent is used to accomplish this, or a solvent.
- The Second Step (Water Rinse) is required to remove dissolved and freed soils and the detergent itself from the metal being cleaned.
- The Third Step (Acid Wash) is to remove free iron, metallic residues, oxides, and other corrosion products from the surface of the metal. By removing these soils from the metal surface and providing an oxidizing atmosphere, the passive film is allowed to form and the passivation is accomplished. Inorganic acids are typically used in this step of the procedure.
- The Fourth Step (Final Water Rinse) – The acidic solution is flushed and the system is rinsed until the quality of the effluent is equal to that of the influent.
Passivation Chemical Alternatives
- Nitric acid, a strong oxidizing acid, is the most common acid specified for passivation. Besides its ability to produce a free iron surface, the acid supplies the oxidizing atmosphere needed for passivation to occur.
- Because nitric acid is a corrosive chemical, extreme care must be used with handling, storage, and use.
- Federal Specification QQ-P-35C (1988) is an excellent reference for obtaining guidelines when using nitric acid on a variety of stainless steel alloys.
- Although nitric acid has traditionally been the preferred passivating acid, the trend in use of passivating solutions is to reduce chemical aggressiveness and to make safety, cost, and the environmental impact of the waste solution effluents a consideration.
- Citric acid and ammonium citrate (ammoniated citric acid) are gaining popularity as alternatives to using nitric acid. The safety these chemicals offer the personnel and the work environment are desirable qualities.
- The ASTM Standard A 380 (1996) refers to these acids as cleaning acids, not passivating acids. This distinction has probably been made because the acids are not oxidizers as is nitric acid.
- The standard states that the citric acid-sodium nitrate treatment is the least hazardous for removal of free iron and other metallic contamination and light surface contamination. To achieve a true oxidation chelating agents in conjunction with citric acid and ammonium citrate has recently been introduced to the pharmaceutical/biotech industry.
- Phosphoric acid is a weak oxidizing acid sometimes specified in passivation procedures; however, there is no formal documentation referencing the use of phosphoric acid as a passivating acid.
- Chelants, otherwise known as sequestering agents or co-ordination compounds, which include all the standard water softening compounds such as Sodium tri-polyphosphate (STPP), Nitrilotriacetic acid (NTA), and Ethylene Diamine Tetra Acetic acid (EDTA) may be compounded into acid passivation solutions to enhance metal ion extraction.
- Orbital welding in conjunction with the increased use of electropolished tubing decreases the aggressiveness required of the passivating acids during the initial passivation. Decreasing acid contact time, temperature, and/or concentration accommodates the quality of the welds and already passive surface of the electropolished stainless steel.
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