Upgrade Your Passivation to Avoid Metal Surface Problems and Manufacturing Delays

Able Electropolishing Company, Chicago, IL

By incorporating the benefits of passivation with surface finish improvement and micro-deburring, electropolishing has become the finish of choice for metal components used in medical devices. This “reverse plating” operation does more than just provide an aesthetic benefit; it provides many functional benefits such as improved corrosion resistance, improved microinch finish, deburring, as well as improved fatigue life. Experienced design engineers anticipate adding the electropolishing operation during the design and prototyping stages. Sometimes though, the finishing operation is an afterthought only added when the product fails to function as designed, often delaying a product’s launch.

Table 1 – Microinch improvement at various stock removals.

The lack of planning for electropolishing operation can result in costly redesigns that could have been easily avoided if given some consideration during the design of the metal components. Three things that are commonly overlooked when designing a part for any electrochemical process are: effect on dimensions, design for rinsing, and determining a fixturing location.

Fig. 1 – Electropolished parts feature a bright finish.

Electropolishing is a process in which material is removed in a controlled manner using a combination of electrical current and chemicals. Electropolishing achieves a deburring effect by removing material preferentially from higher current density areas. Because of this effect, material is not removed evenly from all surfaces, often warranting the processing and analysis of samples when tolerances are critical. While the amount of stock removal can be controlled by experts in electropolishing to +/- .0001", the geometry of the part, the material, and the reasons for electropolishing the part impact how particular features of the part will be affected. Most design engineers seek to limit the amount of material removal during the electropolishing process to allow for tight tolerances. However, this is not always the ideal situation when looking at trying to improve surface finish or when trying to consistently remove burrs. As Table 1 shows, the optimal amount of stock removal to improve the surface finish by approximately 50% is .001". By planning ahead and running samples, parts can be made oversized, or to the high end of the tolerance, and brought down to nominal range with great consistency from part to part and lot to lot.

The electrolytes used in electropolishing are frequently the consistency of maple syrup. This is important to consider in the design of metal components prior to electropolishing. The electropolishing solution can become entrapped in blind holes, welds, and any place where the metal is formed and folded back onto itself. A quick conversation with a metal finishing expert can provide the recommendation that a seam be relaxed or that the use of a through hole might be better if possible. Of course some features cannot be compromised and are critical for design and function. In these cases, steps must be taken to provide thorough rinsing of a medical device component. The use of elevated temperatures, cascading rinses, ultra-high pressure (UHP) water, and ultrasonics can aid in the evacuation of all electrolytes and process waters and provide stain-free and ultra-clean parts.

The bright finish of an electropolished part has the result of highlighting any defects from a previous manufacturing operation, or from where the part was fixtured during the electropolishing process. (See Figure 1) A rack mark will exist on any electropolished part and will vary depending on the material of the part and the material of the electropolishing rack, as well as the shape of the rack. Identifying an area that is less critical can help identify how the part should be fixtured so that the resulting rack marks, which appear as slightly raised areas where the rack shielded the part, are less apparent.

Electropolishing is performed on hundreds of alloys for the medical device industry including 316 stainless steel, cobalt chrome, titanium, niobium titanium, nickel titanium, copper, and aluminum. On devices that design engineers know are going to require passivation to remove free iron, many are choosing to upgrade their passivation process to electropolishing of these components as per ASTM B-912. This satisfies the requirement for passivation, and provides a better functioning part, by removing many of the imperfections left by the stamping, machining, forming, cutting, or drilling operations performed on the component.

This article was written by Tom Glass, President, Able Electropolishing Company, Chicago, IL. For more information, Click Here .