In the fast-paced development of the fastener industry, we may hear situations like this: the stainless steel fasteners you provide have rusted, which is by no means stainless steel. What is the problem? How should we solve it? Can we check the correctness of the chemical composition of the material?
Although the common goal of stainless steel applications is to produce perfect expected results according to the intentions of designers and end-users, in many cases people may feel disappointed with the performance of the material. These disappointments often come from several basic categories, but it is important for us to understand the reasons that may cause this situation. In most cases, mastering some basic knowledge can prevent or significantly improve the occurrence of such problems.
Stainless steel is typically defined as having no defects or defects. Based on this performance, the main function of stainless steel is to build a certain degree of anti fouling or anti-corrosion. The most common stainless steel contains two main non ferrous components: chromium (18-20%) and nickel (8-10.5%). Its conductivity and thermal conductivity are lower than those of carbon steel, and it is essentially non-magnetic. It has higher corrosion resistance than ordinary steel and is widely used due to its ease of shaping various shapes. In the industrial field, the corrosion resistance of stainless steel surface treatment also has many different types of effects.
This article will introduce the importance of passivation, polishing media, and surface profile in the corrosion resistance of stainless steel. In addition, the importance of obtaining the correct technical parameters in structural features will also be demonstrated, with appearance being the main factor.
In order to gain a deeper understanding of this topic, we will focus on two key topics for discussion:
Is passivation of stainless steel very necessary?
The importance of stainless steel surface treatment.
Stainless steel is known as "stainless" due to the thin and hard chromium oxide coating formed in an aerobic environment, which protects the metal from corrosion. If the surface is scratched, this protective film will re form. This means that the surface smoothness of stainless steel plays an important role in its anti-corrosion performance. Despite the repeated emphasis on this topic, people still need to keep it in mind so that we can find ways to improve the performance of various grades of stainless steel materials.
The main requirement for stainless steel is corrosion resistance in specific applications or environments. Choosing a specific "model" and "grade" of stainless steel must first meet the anti-corrosion performance requirements. In addition, mechanical or physical properties should also be considered to meet the overall performance requirements.
Is passivation of stainless steel very necessary?
During processing and machining operations such as forming, turning, and rolling cleaning, particles of iron or tool steel can be embedded into the surface of stainless steel components or contaminate the surface. Although stainless steel has high corrosion resistance, the iron pollution generated during the manufacturing process can cause rust and corrosion. If these pollutants are allowed to remain in the material, these particles may corrode and produce rust or stains on the surface of stainless steel.
Although these particles may only cause surface defects, the substrate remains unchanged and maintains basic mechanical properties. However, even if the mechanical properties of the substrate remain unchanged, stainless steel materials cannot undergo passivation in a pitting state. Traditionally, passivation is considered the standard method for cleaning stainless steel; But in reality, passivation is not a cleaning process at all. The passivation process removes various residual iron from the surface of the parts by using nitric acid and citric acid. According to ISO 16048, the basic fact that chromium oxide films immediately form during the production of stainless steel or stainless steel products should receive special attention. This very thin oxide film can be thickened by passivation.
The thickness of the oxide film is approximately 0.002 μ m. Technically speaking, passivation will not remove any oil or other non-ferrous metal pollutants. And these pollutants are thoroughly cleaned with clean water before the actual passivation process. In March 1983, a message was posted at the Electroplating/Anodizing Forum for Industrial Surface Treatment: "Manufacturers waste thousands of dollars every month, and you only need to perform appropriate cleaning to complete the entire passivation process." In addition to proper cleaning, the use of hard alloy tools can minimize iron pollution in stainless steel. Industrial suppliers can use many hard alloy cutting tools to reduce tool wear, which is beneficial for reducing the occurrence of embedded particles in tool steel.
Bossard Application Engineering is committed to helping our customers reduce part costs. One way to reduce costs is to eliminate non value-added processing processes; Passivation is often one of them. Stainless steel passivation is an expensive process and not environmentally friendly. We suggest checking the customer's application requirements to determine if each specific application truly requires a passivation process.
The typical application of stainless steel passivation is in medical implant equipment or instruments, parts used in the food or pharmaceutical industry, sensor systems, or parts required in clean environments. In addition to the above and other applications that users consider appropriate, there are thousands of applications that do not require passivation.
Remember, any residual carbon can lead to surface defects, but this should not hinder the corrosion resistance of base metals or affect the basic performance of parts, which is important. 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 recommended to use untreated parts instead of passivation treatment.
The main requirement for stainless steel should be anti-corrosion in specific applications or environments. Choosing a specific "model" and "grade" of stainless steel must first meet the anti-corrosion requirements. The anti-corrosion of stainless steel comes from the alloying element chromium.
Rich chromium oxide film naturally forms on the surface of stainless steel. If damaged, the film usually repairs itself. In this case, stainless steel is in a passive state. If the film is damaged, the surface is in an active state.
The Importance of Stainless Steel Surface Treatment
Next, we will refer to EN10088-2 to focus on the importance of polishing. This standard defines the Ra value that directly affects corrosion resistance.
Directional rough polished surfaces with Ra values>1.0 microns will exhibit a dark and dull jungle effect, which will result in the accumulation of chloride ions. It is highly likely to have a potential risk of causing corrosion of the passivation film. On the contrary, a Ra value of less than 0.5 microns will present a clean and clear cutting surface, with minimal accumulation of chloride ions. The simple process of satin polishing provides a good solution for overall fine polishing of the surface and reasonable corrosion resistance.
With the changes in surface treatment, we also found that roughness has a controlling effect on the degree of defects. Rough surfaces (Ra>1.0 microns) are more prone to defects, while smooth surfaces (Ra<0.5 microns) exhibit fewer defects.
Scanning electron microscopy was used to detect stainless steel samples with different roughness levels, confirming that smoother surfaces exhibit fewer defects after accelerated corrosion testing.
When solving surface roughness and defect problems, especially for large external surfaces, different polishing sand particles and strip polishing are usually attempted to achieve the desired effect. In addition, three other precautions should be kept in mind:
Positioning - By polishing in a vertical direction, we can minimize the infiltration of harmful substances and maximize the natural washing effect when rainwater and condensation occur. Sometimes referred to as linear polishing, this polishing is achieved by using approximately 150-180 grit sandpaper to polish vertically. This is one of the most common surface treatment methods for stainless steel in metal processing in the construction industry.
Surface Reflectivity - A smoother surface appears brighter, almost like a mirror in some cases, which may be too idealistic in some designs. In this case, we need to specify a "matte" non directional polishing method, such as glass bead peening.
Large scale effects - When using stainless steel in large-scale engineering, structures such as wall frames, horizontal grooves, and perforations should be avoided, as these will increase the effective surface for accumulating harmful substances.
summary
There are many different types of surface treatment methods for stainless steel. Some of them come from grinding machines, but many are used in subsequent processing processes such as polishing, drawing, shot peening, and etching finishes. In applications using the same grade of stainless steel, polishing or different precision treatments can affect corrosion resistance. The EN10088-2 2K standard provides an explanation that the Ra value should not exceed 0.5 microns, which can be easily achieved by using 240 grit silicon carbide polishing strips. Usually, ISO 3506 standard fasteners can be used for most assembly requirements, while additional requirements can be made for special components with different surface roughness.
On the other hand, passivation is completed after thorough cleaning or removal of oxide scale on the surface of stainless steel. Since the term "passivation" is used to describe operations or processes that are completely different from stainless steel, it is necessary to ensure correct operation to achieve the expected improvement in corrosion resistance of stainless steel.