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Pickling and passivation are both chemical processes used to treat metal surfaces, but they serve different purposes and involve different chemicals. Pickling Pickling is a

But touch screens? Those are manufactured with a controlled roughness because too high of a gloss makes the images hard to see. Additionally, a perfectly smooth screen would be too slippery to accurately interact with the virtual keyboard. This roughness is measurable in micrometers (µm), which are 0.001mm, or microinches (µin), which are 0.000001”.

Previously, surface roughness was calculated by Root Mean Square (RMS), which used the same measure of peaks and valleys but utilized a different formula. RMS is sensitive to larger peaks and valleys, where Ra is not. RMS, or Rq, will mostly appear on older technical drawings as it has been phased out in favor of Ra.

Ultimately, the thickness of the stainless steel also plays a factor in both Ra and electropolishing as the thicker metal is capable of withstanding more processing to achieve better smoothness.

Surface roughness is an often-overlooked dimensional aspect of the manufacturing process. While more focus is generally given to the composition of a part and its strength, or to its measured dimensions and tolerances, a surface that is too rough can result in increased friction and premature failure of a part.

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Stainless steel with a 2B mill finish is a bright, relatively defect-free finish produced by a final cold roll pass using polished rollers. It does not show any grain and has been compared to a “cloudy mirror” in terms of appearance. Since thinner metal is passed through the rollers more times than thicker sheets, the thinner metal usually has a lower Ra and a more uniform finish.

There are many other finishes available, but for bio-pharmaceutical use (such as injectables and otic solutions), an Ra of 0.38 µm (15 µin) with an electropolished surface is usually specified and codified under BPE SF-4 standards.

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While the cost clean and purge a system can quickly add up, there is also the cost of the lost production time to consider as well. Overall, the lower the Ra, the higher-purity production application of the vessel. Not only is it easier to clean, but a smoother finish means that it is easier to empty product.

The Ra can be lowered by employing a combination of chemicals and electricity to carefully dissolve the surface of the steel. This process is known as electropolishing. Only between 5 to 10 µm of material is actually removed, and that is primarily the high peaks of the surface. The results makes the valleys much shallower by comparison. The surface roughness can be reduced by up to 50%.

But, while the roughness allows you to interact with the display, it’s also what allows you to leave smudges on the screen.

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Powder and tablet manufacturers may use a slightly rougher surface, around 0.5 µm (20 µin), as specified under BPE SF-2 standards, which does not require electropolishing.

It should be noted that Surface Roughness differs from Surface Finish. The term “Finish” is used to describe the appearance of a stainless plate or sheet and can be highly subjective. Surface Roughness is objectively measured with calibrated equipment.

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The surface finish of a vessel, as well as its Ra, determine what product can be produced within it, and as stated above, increasing levels of purity require increasingly finer surface finishes with lower Ra numbers. Each industry has specific finish standards that must be met. Sanitary food grade finishes generally fall in the 0.5 µm (20 µin) to 0.7 µm (27 µin) range. This range eliminates places where bacteria or other contaminates can gain a foothold.

We interact with smooth surfaces throughout our everyday lives. Glass windows are smooth, skin is smooth, and the touchscreen on our mobile devices are smooth. Or are they? Anyone who paid attention in biology class knows that, when viewed up-close skin is not smooth.

Ra is measured using a profilometer. This is an instrument with a stylus that travels across the surface and measures the difference in height between the peaks and valleys of the surface profile. ISO standards use the term CLA (Center Line Average), which is interpreted identically to Ra.

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Going beyond mechanical operation, high purity manufacturing requires smooth surfaces within the processing equipment to avoid contamination or build-up within it. Simply put, the smoother a surface is, the less likely it is that material will stick to it.

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Generally, the higher the level of purity demanded in the product, the finer the surface finish that will be required in the manufacturing equipment. For example, a 2B finish is used in baking equipment, food processing, tanks and vessels, pharmaceutical equipment and vacuum drum dryers. It is considered as smooth or smoother than a polished #4 finish and both are acceptable for meeting USDA standards. The Ra for a 2B finish is typically between 0.3 µm (12 µin) and 1 µm (40 µin), depending on the gauge of the metal.

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Electropolishing is not the correct solution for heavily damaged surfaces such as caused by physical impact, welds or chloride micropitting. In those cases, mechanical polishing such as sanding or grinding may need to be employed to reduce the Ra to near the desired range. Once that is accomplished, then electropolishing is performed. While electropolishing delivers an overall smoother surface, it also removes any embedded debris  such as abrasive dust or metal fines that may have been burnished into the surface.

Roughness itself is a series of microscopic “peaks and valleys” across a surface. This becomes clearer when viewed in cross-section. Surface roughness is calculated measuring the average of surface heights and depths across the surface. This measurement is most commonly shown as Ra for Roughness Average and that value is used to determine compliance of equipment with various industry standards.

Since 2012, Mike has served as the Technical Director of Electropolishing at Astro Pak Corporation. With over thirty years of mechanical, machining, and metal finishing experience, Mike brings practical knowledge, capability and excellence to vessel restoration projects across the country. Mike develops and implements training programs, including confined space entry, ensuring technical excellence and safety amongst all Astro Pak MP/EP Technicians.

Formally, Ra is described in ASME B46.1 as “the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length.” Ra, Rq (RMS), Rv, Rp, Rz and some other parameters are two-dimensional in nature, being only concerned with ‘up and down’ measurements in a straight line.

They do not take into consideration other components of the surface topography such as flaws, errors of form, or waviness (symbolized as Sa, Sq, Sz) that would be measured in a 3D evaluation. Two dimensional roughness measurements are usually taken across any grain that might be present.

Additionally, RMS was typically measured in inches while Ra is usually measured in millimeters in most countries other than the USA as most industries now use metric. Many drawings in the USA will show metric measurements with English in parenthesis, i.e., “0.8 (32)”. Also worth noting, the measure of the absolute distance between the highest peak and the lowest valley is shown as Rz.

As mentioned earlier, a smooth surface makes it harder for the product within the system to stick to the sides of a vessel or piping. Similarly, should free iron or other unwanted material be introduced into the system, there is less likelihood that it will become embedded into the metal and become a source of contamination. With high-purity processes, any contamination can spoil an entire batch of products.

To assist with practical applications, use the conversion calculator below to convert Ra values between micrometers (µm) and microinches (µin):