What is the Black Residue Found on Stainless Steel?

It is common to find excessive black debris when wiping a vessel. This occurrence is frequent in mechanically polished containers, particularly when not adequately prepared and cleaned. The staining on the wipe is called polishing debris, composed of ground-in stainless steel particles and abrasive remnants.

The residue forms a thin film that is not usually visible on the surface, and its removal can pose challenges due to fine particles (metallic debris resulting from sanding/polishing, abrasive materials, and other compounds or polymers) that are firmly attached to the surface.

Cleanliness assessment for new equipment, including containers, processing apparatus, and components, is advised before FAT testing or installation. Furthermore, the state of surfaces in new equipment and systems designed for pharmaceutical processing, which fulfill the requirements of ASME-BPE, should be documented.

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Inspection

The inspection and sampling of mechanically polished surfaces are carried out using a wipe and alcohol within a clean environment. Initially, gently scrub the area with an alcohol-dampened cloth, applying light to moderate pressure, covering an area of approximately one square foot.

Following this, visually inspect the cloth and examine it under magnification to assess the remnants' initial severity and physical traits. The remnants may appear white to light gray or dark gray to black.

In cases of excessive residue, the wipe can be subjected to advanced analytical methods to identify the metals or organic compounds present. Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) and Energy Dispersive X-Ray (EDX) are used to determine the inorganic elements (metal oxides) found on the cloth due to surface residue.  

Fourier Transfer InfraRed spectroscopy (FTIR) is employed to identify any organic compounds within the remnants.

Testing Results

The testing results of vessels over the past five years are outlined below, illustrating the common components discovered in the residue. These tests were frequently prompted by rouge on the vessel's surface or an excessively colored wipe taken from the equipment surface.

The analyses of the wipes are summarized in Attachment "A," arranged from highest to lowest average concentrations. The potential risks associated with this film on the surface can be categorized as hindering the cleaning and passivation treatment, possibly entering the process or product fluids, or acting as a source of rouge and corrosion products.

The elemental analyses lead to the following conclusions. The most prevalent elements (iron and chromium) originate from stainless steel particles remaining on the surface due to polishing (sanding operations).

The second most prevalent group (calcium, sodium, potassium, and magnesium) of elements comes from process fluids or water. Silica (silicates) is present at a comparable concentration level and is one of the more commonly used abrasive media components in polishing operations.

Other abrasive elements, including aluminum, are observed at lower concentrations. The final group of elements encompasses manganese (often present on the surface of stainless steel) and metals found in small amounts in stainless steel (like copper, molybdenum, zinc, and titanium).

Occasionally, phosphorus is detected on the surface (usually from cleaning agents or the stainless steel itself). Carbon is not quantitatively assessed, given that the wiping material primarily consists of carbon. Organic analyses of the surface residues reveal a minimal presence of oils, greases, waxes, esters, phthalates, and various polymers.

These compounds can arise from polishing processes and some originate from the actual wiping material. Throughout all the tests, a clean section of the wipe is included for comparison alongside the colored or residue-laden section.

Due to the low (ppm) levels of contaminants, both the variance in the blank analyses and the variability in the residue analyses lead to qualitative results.

Remediation Techniques

Various methods have been employed to remove these films, including manual wiping with an alkaline cleaner, electrochemical cleaning (flash electro-polishing), and high-pressure washing with chemistries designed for particulate removal. The outcomes vary depending on the surface condition and the method utilized.  

Electro-polishing has proven effective in eliminating surface contamination and the damage layer commonly associated with mechanically polished surfaces. Manual wiping removes most of the residue, although it may require multiple attempts with intermittent hot water washing cycles.

Pressure washing or applying particulate removal chemistries generally yields modest success unless combined with additional mechanical cleaning efforts.

Summary

Analyzing wiped samples from product contact surfaces serves as a method to identify characteristics of potential residue or contamination.

The residue from these wipe samples indicates that it primarily consists of stainless steel particles and oxide compounds resulting from mechanical polishing, along with lower levels of abrasive compounds and remnants of water or process fluids.

Each project presents varying levels of contaminants based on existing conditions and the polishing or cleaning techniques employed on the equipment surfaces. Examining and testing the residue will pinpoint the source and guide the appropriate treatment technique to employ.

Table 1. ICP-MS, EDX, FTIR. Source: Astro Pak Corporation

This information has been sourced, reviewed and adapted from materials provided by Astro Pak Corporation.

For more information on this source, please visit Astro Pak Corporation.