Surface Treatment of Stainless Steel Fasteners and Components
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2024-11-04
Amid the rapid growth of the fastener industry, you may encounter situations like this: the stainless steel fasteners you supplied have rusted—there’s no way they’re truly stainless steel. What went wrong, and how can we resolve it? Could we verify whether the material’s chemical composition is correct?
Although the common goal of stainless steel applications is to deliver the intended, flawless results as envisioned by designers and end users, in many cases people end up disappointed with the material’s performance. Such disappointments typically stem from a few fundamental categories; however, it is crucial to understand the underlying causes. In most instances, acquiring a basic understanding can help prevent these issues or significantly mitigate their occurrence.
Stainless steel is generally defined as being free of flaws or defects. Based on this characteristic, its primary function is to provide resistance to staining and corrosion. The most common types of stainless steel contain two major non‑iron elements: chromium (18–20%) and nickel (8–10.5%). It exhibits lower electrical and thermal conductivity than carbon steel and is essentially non‑magnetic. With superior corrosion resistance compared to ordinary steels, it is widely used due to its ease of forming into various shapes. In industrial applications, surface treatments designed to enhance the corrosion resistance of stainless steel can yield a variety of distinct effects.
This paper discusses the importance of passivation, polishing media, and surface topography in enhancing the corrosion resistance of stainless steel. Furthermore, it argues for the critical role of selecting appropriate process parameters to achieve desirable microstructural characteristics, with surface appearance being a key factor.
To gain a deeper understanding of this topic, we will focus our discussion on two key aspects:
- Is stainless steel passivation really necessary?
- The importance of surface treatment for stainless steel.
Stainless steel is called “stainless” because, in an oxygen-rich environment, a thin, hard chromium oxide passivation layer forms on its surface, protecting the metal from corrosion. If the surface is scratched, this protective film reforms automatically. This underscores the critical role of surface finish in determining stainless steel’s corrosion resistance. Despite repeated emphasis on this point, it remains essential to keep it top of mind so that we can identify ways to enhance the performance of stainless steels across different grades.
The primary demand for stainless steel is corrosion resistance in specific applications or environments. Selecting a particular “grade” and “class” of stainless steel must first meet the required corrosion‑resistance performance. In addition, mechanical and physical properties must be taken into account to satisfy overall service‑performance requirements.
Is stainless steel passivation really necessary?
During processing and manufacturing operations such as forming, turning, and roll‑cleaning, iron or tool‑steel particles can become embedded in the surface of stainless‑steel components or contaminate it. Although stainless steel exhibits excellent corrosion resistance, iron contamination introduced during fabrication can lead to rusting and corrosion. If these contaminants are left in the material, the particles may initiate corrosion, resulting in rust spots or discoloration on the stainless‑steel surface.
Although these particles may only cause surface imperfections, the substrate itself remains unchanged and retains its fundamental mechanical properties. Nevertheless, even when the substrate’s mechanical properties are unaltered, stainless steel cannot achieve passivation under pitting conditions. Traditionally, passivation has been regarded as the standard method for cleaning stainless steel; in reality, however, passivation is not a cleaning process at all. The passivation process removes various residual iron contaminants from the part’s surface by means of nitric acid and citric acid. According to ISO 16048, particular attention should be paid to the fundamental fact that, during the manufacture of stainless steel or stainless‑steel products, a chromium oxide film forms immediately. This extremely thin oxide layer can be further thickened through passivation.
The oxide film thickness is approximately 0.002 μm. Technically, passivation does not remove any oil or other non‑ferrous contaminants; these are eliminated through thorough rinsing with clean water prior to the actual passivation process. In March 1983, at an electroplating/anodizing forum on industrial surface treatment, it was reported: “Manufacturers waste thousands of dollars each month—yet a proper cleaning step alone would suffice to complete the entire passivation process.” In addition to adequate cleaning, employing carbide tools can minimize iron contamination in stainless steel. Industrial suppliers offer a wide range of carbide cutting tools that reduce tool wear, thereby helping to lower the incidence of embedded particles in tool steels.
One way to reduce costs is to eliminate non-value‑adding manufacturing processes; passivation is often one of them. Stainless steel passivation is an expensive operation and is not environmentally friendly. We recommend evaluating the customer’s application requirements to determine whether passivation is truly necessary for each specific use case.
Typical applications of stainless steel passivation include medical implant devices and instruments, components used in the food and pharmaceutical industries, sensor systems, and parts required for use in cleanroom environments. In addition to these, and other applications deemed suitable by users, there are thousands of applications that do not require passivation.
It is important to remember that any residual carbon can lead to surface defects, but this should not compromise the corrosion resistance of base metals or affect the part’s fundamental performance. The cost difference between passivated and non‑passivated parts is approximately 15% to 20%. If the customer’s application does not require passivation, it is advisable to recommend using untreated parts as an alternative to passivation.
The primary demand for stainless steel lies in its corrosion resistance under specific applications or environmental conditions. Selecting a particular “grade” and “class” of stainless steel must first and foremost meet the required corrosion‑resistance criteria. The corrosion resistance of stainless steel stems from the alloying element chromium.
A chromium-rich oxide film forms naturally on the surface of stainless steel. If damaged, this film typically repairs itself. Under these conditions, the stainless steel is in a passive state. However, if the film is compromised, the surface becomes active.
The Importance of Stainless Steel Surface Treatment
Next, we will examine the importance of polishing in accordance with EN 10088‑2. This standard specifies the Ra value, which directly affects corrosion resistance.
A directional, coarse‑polished surface with an Ra value greater than 1.0 µm will exhibit a dark, matte “jungle” appearance, which can lead to the accumulation of chloride ions and pose a potential risk of pitting corrosion. In contrast, an Ra value below 0.5 µm yields a clean, sharply defined cut surface, with negligible chloride‑ion buildup. This straightforward satin‑finish polishing process offers an effective solution for achieving an overall finely polished surface while ensuring adequate corrosion resistance.
As surface treatment varies, we also observe that surface roughness exerts a controlling influence on the severity of defects. Rough surfaces (Ra > 1.0 μm) are more prone to defects, whereas smooth surfaces (Ra < 0.5 μm) exhibit very few defects.
Scanning electron microscopy was used to examine stainless steel samples with varying degrees of surface roughness, confirming that, following accelerated corrosion testing, smoother surfaces exhibited fewer defects.
When addressing surface roughness and defects—particularly on large exterior surfaces—it is common to experiment with different polishing grits and belt‑polishing techniques to achieve the desired results. In addition, three other considerations should be kept in mind:
- Polishing in the vertical direction—also known as linear polishing—minimizes the penetration of contaminants while maximizing the natural self‑cleaning effect during rainfall and condensation. This process is typically carried out using sandpaper with a grit of approximately 150–180, applied in a vertical motion. It is one of the most common surface‑finishing techniques for stainless steel in the construction industry.
- Surface reflectance – Smoother surfaces appear brighter and, in some cases, almost mirror-like, which may be overly idealized in certain designs. In such instances, it is necessary to specify a “matte” non‑directional finishing process, such as glass bead blasting.
- Mass‑effect considerations – When using stainless steel in large‑scale projects, avoid features such as ledges, horizontal grooves, and perforations, as these can increase the effective surface area for the accumulation of harmful substances.
Summary
Many different surface‑treatment methods have been developed for stainless steel. Some originate from grinding processes, while others are applied during subsequent operations such as polishing, brushing, shot peening, and etching. In applications involving the same grade of stainless steel, the choice of polishing or other finishing techniques can influence corrosion resistance. The EN 10088‑2 2K standard specifies that the Ra value should not exceed 0.5 µm, a requirement that can be readily met using a 240‑grit silicon carbide polishing belt. Typically, fasteners conforming to ISO 3506 are suitable for most assembly requirements, though additional specifications may be imposed for special components with particular surface roughness characteristics.
On the other hand, passivation is carried out after the stainless steel surface has been thoroughly cleaned or descaled. Since the term “passivation” is used to describe operations or processes that are entirely distinct from those applied to stainless steel, it is essential to ensure proper procedures are followed to achieve the intended improvement in the material’s corrosion resistance.
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