Previous Issues

by Daryl L. Roll, P.E.

Have you ever wiped a vessel and found that the wipe had excessive black debris? This is common on mechanically polished vessels, especially when they have not been properly prepped and cleaned.  The discoloration on the wipe is classified as polishing debris consisting of ground-in stainless steel particles and abrasive residue. The residue is a thin film not generally visible on the surface and its removal can be problematic since it consists of very fine particles (metallic debris from sanding/polishing, abrasives, and other compounds or polymers) that adhere to the surface.

Cleanliness evaluation of new equipment including vessels, process equipment and components is recommended prior to FAT testing or installation. Additionally, the condition of surfaces in equipment for pharmaceutical processing compliant with ASME - BPE requirements should be documented.

Inspection

Inspection and sampling of mechanical polished surfaces is performed with the use of a clean room wipe and alcohol. First, scrub the surface with light to moderate pressure using an alcohol wipe over approximately one square foot area. Next, inspect the wipe visually and/or under magnification to initially evaluate the severity and physical characteristics of the residue.  The residue can appear white to light gray in color or, more commonly, dark gray to black.

If residue is observed, the wipe can be further evaluated with advanced analytical techniques to identify the metals or organic compounds present.  ICP-MS (Inductively Coupled Plasma-Mass Spectrometry) and EDX (Energy Dispersive X-Ray) are used to identify the inorganic elements (metal oxides) present on the wipe from the surface residue. FTIR (Fourier Transfer InfraRed Spectroscopy) is used to identify any organic compounds.

Testing Results

The testing results of vessels over the past 5 years are presented below to describe the typical components found in the residue. These tests were often the result of the appearance of rouge on the surface of the vessel or the appearance of a wipe from the equipment surface that was considered excessively colored.  The analyses of the wipes are summarized in Table 1, from highest to lowest average concentrations.  The risk of this film on the surface can be categorized as inhibiting the cleaning and passivation treatment, potentially available for release into the process or product fluids or as a source of rouge and corrosion products.

The elemental analyses lead to the following conclusions. The most prevalent elements (iron and chromium) are from the stainless steel particles present on the surface from polishing (sanding) operations. The second most prevalent group of elements (calcium, sodium, potassium and magnesium) is from process fluids or water. Silica (silicates) is found at a similar level of concentration and is one of the more common abrasive media components used in polishing operations. Other abrasive elements including aluminum are also seen at lower concentrations.  The last group of elements includes manganese, which is often present on the surface of stainless steel, and metals found in low concentrations in stainless steel such as copper, molybdenum, zinc and titanium.  Phosphorus is sometimes found and generally originates from cleaners or from the stainless steel. Carbon is not quantitatively evaluated since the wiping material is largely composed of carbon.

Organic analyses of the surface residues show a low level of oils and greases, waxes, esters, phthalates and assorted polymers. These are compounds that can be found in polishing processes or in the wipe itself. As in all this testing, a blank (clean) part of the wipe is processed in addition to the colored or residue-laden section for comparison. (Note: The variance in the blank analyses, combined with the variability in the residue analyses due to the low (ppm) levels of contaminants, means only qualitative results are presented.)

Table 1 - Analysis of Wipe Sample

Very High   -                      20%  ± 15%
High           -                      15% ± 10%
Moderate    -                      10% ± 5%
Low            -                      5% ± 5%
Very Low   -                      2% ± 2%
Trace          -                      1% ± 1%

Remediation Techniques

Removal of these films has been attempted in a number of ways, including hand wiping with an alkaline cleaner, electrochemical cleaning (flash electropolishing) and high pressure washing with particulate removal chemistries. Results vary based upon the condition of the surface and the technique employed. Electropolishing of the surface has been shown to effectively remove both the surface contamination and the damage layer often associated with mechanically polished surfaces. Hand wiping removes the majority of the residue, but can require multiple treatment efforts with hot water washing cycles between wiping efforts. Pressure washing or the use of particulate removal chemistries is generally only marginally successful, unless combined with additional mechanical cleaning efforts.

Table 2 - Removal of Debris from Mechanical Polishing

Summary

Analysis of wiping of product contact surfaces is a method to determine characteristics of potential residue or contamination. The residue from wipe samples show that it is mostly stainless steel particles and oxide compounds generated from the mechanical polishing with lower levels of abrasive compounds and water or process fluid residues. Each project presents a different level of contaminants based on existing conditions and polishing or cleaning techniques employed on the equipment surfaces. Sampling and testing of the residue will indicate the source and guide you in the proper treatment technique to be used.

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