Electropolishing: Stainless Steel Fasteners in Corrosive Environments

In the world of fastener applications, we can find a wide range of use cases that share the common interest of corrosion resistance and easy cleanability.  Applications in the medical, food and beverage, semiconductor, pharmaceutical, marine, and chemical industries make up the core users of highly corrosion-resistant, cleanable hardware.  Over the last several decades, buyers in these fields have continuously presented hardware manufacturers with increasingly stringent requirements, where now today we routinely receive hardware specifications requesting the following features in a single fastener: 

• Corrosion resistant against strong chemicals (caustic and acid solutions over 10% concentrations).
• Corrosion resistant against mineral attack such as from seawater.
• Long life in daily aggressive exposure conditions (10+ years).
• Ability to withstand sterilizing temperatures (over 250*F).
• Very low surface roughness (under 20 RA micro inch).
• High rinseability / easy water sheen.
• No applied finish that can chip or flake off (which presents a potential contamination risk to nearby sensitive goods, such as around food preparation).

Seeing a specification list such as the above immediately leads us to one clear solution: stainless steel fasteners with an electropolished finish.   

Stainless steel fasteners on their own provide a high degree of corrosion resistance on account of their low carbon, high nickel composition.  This feature alone makes SS 304 and 316 alloys the preferred choice in most aggressive applications, where daily chemical exposure and / or sterilization occur.  Users will further select a surface finish roughness value – denoted as RA and measured in micro inch units – where a lower roughness value describes a smoother surface, reducing the risk of microscopic harborage and increasing how easily foreign materials will slide off of the surface.    

A ‘mill’ finish SS fastener will have a rough finish over 100 RA, with machining or casting marks clearly visible.  Improving this finish quality to the next tier involves applying a mechanical polish to the fastener, typically performed with polish machining passes or abrasive tooling, which can achieve a finish in the 30-40 RA range.  Pushing ahead to an even higher finish tier with advanced polishing and grain stripping can achieve an 18-25 RA finish range.  At this point, mechanical methods of improving surface conditions hit their functional limit, leading us to electropolishing processes as our path to even higher quality surface finishes.     

Do you have a Corrosion Resistant Fastener application but are unsure of which fastener meets your project’s requirements?  E&T Fasteners offers expert support and product selection between the above options and more!  Contact us to discuss your application, see our Screw and Bolt catalog here, or to see our entire fastener catalog, visit our product selection here. 

Electropolishing in Action

In most practical manners, a surface roughness of 0 RA is not possible.  That said, electropolishing allows us to get as close as we’re able to with mature manufacturing processes, down to values between 5-6 RA.  At this roughness level, stainless steel fasteners offer the lowest viable level of contaminant harborage, while gaining heightened corrosion resistance due to the electropolishing process.  Let’s describe that process next, and the benefits after that.   

Electropolishing is performed by submerging stainless steel fasteners into a fluid bath made up of sulfuric and phosphoric acids.  This fluid is brought to a specific temperature, serving as a conductive electrolytic medium.  DC electrical current is energized across the electrolytic solution and the stainless steel parts, which produces an aggressive reaction across the entire surface of the parts.  A very thin, controllable layer of the parts’ surfaces are dissolved away into the acid bath, leaving behind an extremely smooth, highly polished finish.  Generally, surface material is removed between 0.001″ and 0.003″ in thickness, adjustable by the amount of time parts are left under current.  Surfaces are not dissolved evenly – instead, high material features are removed first, equalizing features down to the lowest common dimension smoothly.

Once complete, electropolishing leaves us with bright, highly polished, very smooth stainless steel fasteners.  Let’s list out the full benefits of electropolishing here:

• Easier cleanability by smoothing out texture and ridge features where undesirable contaminants may collect.
• Tighter dimensional tolerances achieved through dissolving away high surface features.
• Maximizes corrosion resistance by dissolving away residual byproducts of the fasteners’ manufacturing process, most notably free iron, machining lubricants, and handling fibers (such as from employee gloves or workbench mats). 
• Attractive, bright, mirror-like finish.
• Inherently provides complete passivation to the parts during electropolishing.  (Passivation is a chemical reaction often performed on stainless steels to replace the naturally-occurring passive oxide layer covering the materials surface, which is what gives stainless steel is great corrosion resistance.) 
• Removes scale, burrs, and other manufacturing remnants without the use of an additional mechanical finishing step. 
• Maintains and exceeds common regulatory and code requirements where they apply, such as with 3A Standards for food process applications, or ASTM A967 for chemical passivation applications.

E&T Fasteners is here to help you navigate fastener and accessory options.  Price quotes, availability, lead times, delivery confirmations, unique application suggestions – whatever your need, we’re here to take your call or email.  For support and advice from our fully trained staff, speak with an E&T sales representative today. 

About E&T Fasteners

With sales and support teams spanning the Continental US, E&T Fasteners is here to serve your hardware, fastening, and component needs both domestically and internationally.  We are a stocking distributor of metallic, plastic, and exotic alloy fasteners, backed by our technical staff directly experienced in your unique applications.  We can assist your specifying and selection efforts, providing engineered solutions, record-fast lead times, and stellar customer service from quote through delivery.  Please let us know how we can help!