Enter Stainless Steel

Stainless steel provides excellent corrosion resistance and withstands high temperatures — two characteristics that make it an ideal material for components that must withstand movement and high levels of friction. It better resists corrosion than aluminum and other steels, and it is less expensive than titanium and cobalt alloys.

The four types of stainless — austenitic, ferritic, martensitic, and precipitation hardening — vary in their respective levels of corrosion resistance, with austenitic providing the greatest resistance. Each type serves its own purpose in various applications. Martensitic, for example, is an excellent choice for surgical instruments because of its hardness, whereas ferritic is not because it cannot be hardened by heat treatment and can only be moderately hardened by cold working.

What Exactly is Stainless Steel?

Adding at least 12% chromium to steel classifies it as stainless. That’s because the chromium, when combined with the oxygen in the atmosphere, forms an invisible oxide layer. When scratched, the oxide layer naturally reforms. As a result, the steel resists rust and abrasions and, by extension, reduces the chance of contamination. This last attribute makes stainless steel well-suited for the culinary and medical industries; the former attributes allow for its use in bearings, aircraft landing gear, and other high-friction applications.

NiCoTef® Reinforces Stainless

Even stainless steel can experience wear and galling due to factors like friction and surface hardness. These conditions lead to surface degradation that can produce particles that in turn can contaminate their environment. In the medical and food industries this is unacceptable.

That’s where Nimet Industries comes in. Nimet’s engineers work with engineers in the medical, pharmaceutical, and aerospace industries, among others, to identify performance needs in bare-contact materials and to develop solutions. Precision metal finishes like NiCoTef® that use Teflon® function as a dry lubricant and increase surface hardness, ultimately increasing wear resistance. NiCoTef® has been approved by the FDA in countless applications. More information about NiCoTef® is available on our website.

Case in Point

Engineers from a major manufacturer of laser lithography equipment used in the production of semiconductors contacted Nimet about a problem they were having with galling. Most of their machine components are made from stainless steel and are assembled robotically in a clean room environment. The threaded stainless fasteners were galling during installation, causing some to break and others to overload the robot because of friction, which caused the line to shut down. Because of sanitation requirements, the engineers had to find a surface treatment that would reduce friction without contaminating the environment; liquid and dry lubricants were out of the question.

Nimet had the solution that met all the requirements: NiCoTef®.

NiCoTef® has reduced the manufacturer’s costs in a number of areas. Since implementing NiCoTef® treatment, the manufacturer has eliminated all of their assembly line issues and now uses NiCoTef® on all of their fasteners. The cost to coat with NiCoTef® is a fraction of what the downtime had cost, and NiCoTef’s corrosion protection enabled the manufacturer to convert many of the fasteners from stainless steel to less expensive steel alloys.

In hardcoat anodizing, the current density is higher, the operating temperature of the electrolytic bath is lower, and the resulting coating is more dense than in traditional anodizing. Of course, as with any process a number of variables influence the final product. Planning for the different variables provides greater control, which can reduce manufacturing costs.

What variables affect coating quality and dimensional tolerances?

Each processing stage includes a number of variables that can be controlled to maintain close tolerances and coating integrity.

Design Stage

• Aluminum alloy
• Fabrication method
• Accounting for dimensional changes that occur during anodizing

Processing Stage

• Current density
• Electrolyte concentration
• Temperature
• Coating thickness

Alloy Composition Affects Density & Hardness

In anodizing, the coating is generated from the aluminum substrate, which means that the alloy’s quality and composition have a direct impact on the quality of the anodic coating. In general, high-purity or commercially pure aluminum produces denser and harder coatings than heat-treated alloys. The latter contains alloying elements that may not dissolve during the anodic process, which may cause microscopic voids that reduce density and ultimately resistance to corrosion and abrasion. When choosing an alloy, keep in mind that alloys with low copper and low silicon content as well as a few cast alloys yield the best coating.

Holding Dimensional Tolerances

That aluminum oxide coating is less dense than the aluminum substrate means that both growth and penetration occur. Coating adds to the original surface, so machining dimensions should be adjusted before coating to account for the change in size. Standard coating on most alloys is 0.002 inches, half of which forms below the surface and half of which is added.

Die castings are an exception. Their surface includes a high silica content that inhibits the growth of anodic film and as a result yields a thinner coating.

What variables affect coating color?

• Chemical composition and temper of the alloy
• Coating thickness
• Aluminum fabrication prior to processing (e.g. Areas that have been machined or welded may have different tempers than the rest of the surface.)

What are the best surfaces for anodizing?

Hardcoat anodizing is well-suited for machining methods like milling, turning, and drilling because of its ability to withstand the abrasive techniques used in these processes. It’s critical to follow guidelines for coating specific parts like holes, corners, and edges and to calculate tolerances correctly.

Nimet Industries uses PTFE technology in the Nituff® process, which combines PTFE with aluminum oxide to form a coating for aluminum and its alloys. The result: a self-curing, self-lubricating surface with wear and corrosion properties superior to standard hardcoat anodizing. In the Nituff® process, Nimet electrochemically oxidizes the outer surface of the aluminum substrate to grow crystals of aluminum oxide, which become an integral part of the structure.

Before the crystals are completely formed, sub-micron particles of Teflon® PTFE are introduced into the aluminum oxide matrix. The coating is .002 inch thick, .001 inch below the surface and .001 inch above. This hard, abrasion-resistant coating has a non-conductive surface with breakdown voltage exceeding 2,000 volts.

Combined with traditional metal anodizing or plating, PTFE adds corrosion protection and self-lubrication, among other properties, to base metals. In corrosion-resistance testing, the Nituff® coating survives more than 2,000 hours in a five percent salt spray as compared to the 850 hours typical for hardcoat anodizing alone.

PTFE at Nimet Industries

Nimet enlists PTFE composite coatings to serve industries from dental to defense. USDA-approved Nituff® is used to treat medical, dental, and food processing equipment to provide self-lubrication without oils or greases. It also provides abrasion resistance in components used in high-speed machinery that handles textiles or paper; its dry lubrication properties eliminate the need for lubricants that could stain paper or textiles.

Fluoropolymers coatings can also be used with other metals and in electroless nickel processes. Electroless nickel PTFE coatings like Nimet’s NiCoTef® are used in a variety of ways.

• To repel water, oil, and dirt
• To enhance the release of properties of molds for plastic and rubber components
• To provide dry lubrication and reduce friction
• To improve wear resistance
• To increase corrosion protection

Nimet offers other combinations of metal surface treatments and penetrated fluoropolymer particles. Contact us with specific needs or questions.

Substrates often used in the EN process include steel, aluminum, and non-metallics.

Varieties of EN plating generally fit into one of three categories based on the amount of phosphorus deposits present: low, medium, and high. The level of phosphorus determines how hard and how well a deposit resists corrosion. High levels of phosphorus deposits yield high corrosion resistance, but are softer than low phosphorus deposits. The opposite is also true: deposits with low levels of phosphorus are harder but less resistant to corrosion than deposits with high levels of phosphorus.

Characteristics of electroless nickel plating

EN chemistry has a consistent, controllable coating deposition rate, which enables close tolerances — within +/-1 to 2 microns — to be maintained. Performance needs determine the combination of characteristics such as hardness, wear/abrasion/erosion and corrosion protection, protection from chemical attack, lubricity, solderability, and high temperature performance.

EN plating’s excellent wear characteristics (e.g. hardness, lubricity, low coefficient of friction, and uniform coverage capability) make it a perfect fit for a range of applications and problem-solving tasks. Under lubricated conditions, EN deposits can equal or surpass the results typically achieved by hard chromium plating, which is particularly important when utilizing heavy deposit build-ups in overhaul and repair operations.

What role does electroless nickel plating serve?

Like hardcoat anodizing, EN coatings improve wear and corrosion resistance, and they can also improve aesthetics. With applications in the chemical processing, food processing, pharmaceutical, medical, and dental industries EN plating is used in items such as heat exchangers, carburetors, and electronic components. Its resistance to the corrosion often caused by gasoline-alcohol mixes makes it a logical choice for the auto industry when plating fuel injection systems and carburetors and for pipeline components used in the oil and gas industries.

EN can be done through barrel plating and in bulk, which enables Nimet Industries to save customers the costly expense of racking small parts.

Selective plating is exactly what it sounds like.

Specified areas on a part are “masked off” from the plating process.

Typical features that may require masking are:

• Threaded holes
• Dowel pin holes
• Electrical contact points
• Dissimilar metals on a single substrate
• Tightly toleranced features

Properly utilized, the masking procedure will achieve the desired results.

Masking materials include:

• Liquid masking chemicals
• Plugs
• Plastic fasteners
• Tape
• Die cuts

But there are potential pitfalls, and masking does add time and cost to the plating process.

The masking process is seldom foolproof and may come with its own set of potential problems. For example, masking polished surfaces is difficult because the masking material requires a degree of surface roughness for a mechanical bond. A good analogy would be the difficulty of getting paint to adhere consistently to a high-gloss finish. Masking on these surfaces has a tendency to lift off at the edges. Plug masking, the most straightforward solution, also can pose challenges as plugs can and do leak, or even fall out during the plating process.

Parts designers should endeavor to avoid the time and expense of masking whenever possible. One way to accomplish this is to be sure to make allowances for coating build-up when the part is machined. Another option is to allow for a secondary machining operation to remove the coating in the selected areas.

If you have questions about masking, the Nimet engineering staff is available to discuss the best solutions. And our masking department has the training and experience to assure the best possible outcome for your selective plating needs.

Summary of “Synergistic Fluoropolymer Coatings”

Synergistic fluoropolymer coatings are the result of infusing a low-friction fluoropolymer like Teflon into an aluminum oxide, which makes the fluoropolymer a major ingredient in the top coating rather than establishing it as another layer in and of itself. What this means is a number of advantages in the final product:

• Self-lubrication
• Greater corrosion resistance
• A low coefficient of friction
• Increased hardness (Rockwell C62)
• Chemical resistance
• Improved dielectric properties

Nituff® is such a coating. It combines polytetrafluoroethylene (PTFE) with aluminum oxide and ultimately yields a surface far superior to standard hardcoat anodizing.

Process Overview

An aluminum surface that will be treated undergoes these steps:

1. The aluminum surface is electrochemically oxidized to stimulate growth of aluminum oxide crystals. (This is part of the standard anodization process.)
2. Sub-micron particles of PTFE are introduced. (The timing of this introduction is critical: crystals must be only partially formed.)
3. Completely formed crystals have trapped the PTFE particles, integrating them into the coating. (This forms the final coating.)

The coating that results, as noted above, has a Rockwell hardness of C62. It also has a nonconductive surface that withstands more than 2,000 volts and more than 2,000 hours in a 5% salt spray, where typical hardcoat anodizing withstands only 350 hours in a 5% salt spray. Such a coating has many applications, such as military and culinary, because its non-reflective surface also repels dirt.

Synergistic Fluoropolymer Coatings at Nimet Industries

Nimet Industries has been in the business of precision metal finishing for more than 45 years. We have seen first-hand the benefits that our proprietary process afford our customers, and we encourage you to contact us with questions or to discuss how we can put our expertise to work for you.

Clear anodizing

Also known as sulfuric acid anodizing, clear anodizing provides corrosion protection in mild environments and minimum wear protection. Compared to chromic acid anodizing, sulfuric acid anodizing can provide similar corrosion protection and better wear characteristics. Specification determines the coating thickness.

Process

Sulfuric acid anodizing yields a porous film that readily absorbs dyes. The basics process involves:

1. Sulfuric acid process anodizes the aluminum substrate
2. Cold water rinse
3. Dye
4. Cold water rinse
5. Hot water bath to seal the anodic film

Uses

Clear anodizing’s relatively low cost makes it a commonly requested specification on engineered aluminum components. It is used in many architectural applications (e.g. windows, siding, and railings). It is often dyed for decorative purposes.

Hardcoat anodizing

Hardcoat anodizing yields a thicker, denser oxide coating than clear anodizing.In hardcoat anodizing the film thickness ranges from three to 20 times that of clear anodize, its density producing a hardness of 60 to 65 Rockwell on the C scale. The material’s alloy and temper along with the coating thickness determine the color of film – from light gray to dark olive gray – generated from the hardcoat process.

Hardcoat anodize is also dielectric (does not conduct direct current), so may be used to insulate assembly components.

Process

The difference between hardcoat and clear during the same basic process is the recipe of the chemical substrate along with the processing voltage.  The process involves the same steps:

1. Sulfuric acid process anodizes the aluminum substrate
2. Cold water rinse
3. Dye
4. Cold water rinse
5. Hot water bath to seal the anodic film

Uses

More expensive than clear anodizing, hardcoat anodizing is often more economical than chromic acid anodizing in terms of the wear resistance achieved. Hardcoat anodizing is often sought for components that require high wear and corrosion resistance. It is typically applied to machine wrought aluminum and is used in hydraulic and pneumatic components, screw threads, heat sinks, and has many other applications.

Understanding a specification: MIL-A-8625

To give you an idea of what a specification involves, let’s take a look at MIL-A-8625, a common anodizing specification in the United States that outlines three types of aluminum anodization:

Type I: Chromic Acid Anodization

A chromic acid solution yields a thinner, more opaque film that is softer and more pliable than other solutions. This type divided (e.g. Type IA and IB) that describe variations such as voltage requirements.

Type II: Sulfuric Acid Anodization

A sulfuric acid solution that yields a coating with a thickness between 0.00007″ and 0.001″ is considered Type II.

Type III: Sulfuric Acid Hardcoat Anodization

A sulfuric acid solution that yields a coating with a thickness great than 0.001″ is considered Type III. This type further breaks down into classes: Class 1 indicates non-dyed and Class 2 indicates dyed.

The tests that the above types must pass assess oxide coating thickness, oxide coating weight and apparent density, corrosion resistance, and seal quality.

Nimet’s proprietary processes take anodization to a higher level

At Nimet Industries, we often employ the above processes to anodize aluminum, and we’ve also developed our own processes to attain an even higher level of performance:

• Nituff® penetrates fluoropolymers into precision coatings for metals; Black Nituff® does, too, but has a rich, black finish. Both meet specification MIL-A-8625 Type III.
• Nituff C® is the clear version of Nituff® that enables us to dye to customers’ specifications. Nituff C meets specification MIL-A-8625 Type II.
• NiCoTef® is an electroless nickel composite coating that uses 25% Teflon to reduce friction.
• Anografics® embeds photorealistic four-color images on aluminum surfaces

Our proprietary coatings mean that lower-cost materials perform as well as or better than traditional designed components. For more information about applications visit the Products section of our site.

Now that we’ve explained what it is, here are some reasons why anodizing is so beneficial:

• Extremely durable – abrasion and corrosion resistant
• Won’t chip, flake, or peel
• Inexpensive to produce and to maintain

The various anodizing finishes each provide the above benefits along with others:

• Clear anodic finish allows the aluminum substrate to show through.
• Hardcoat anodic finish produces a more dense coating and is more wear-resistant.
• Dyed anodic finish offers unlimited color options.

Applications of Anodizing

Nimet’s products (like Nituff®) and processes serve a variety of engineering applications in a variety of industries. Examples include:

• Aircraft and aerospace (e.g. aircraft brake components, pressure regulating valves for astronaut breathing apparatus, and missile actuators
• Decrease cleaning time (e.g. spray paint applicators, soldering nests, and military night visioning hardware)
• FDA-approved medical, dental, pharmaceutical, and food processing (e.g. non-stick cooking services)
• Textiles and paper (e.g. machines’ spools, rolls, and guides)

Up next: anodizing processes.

Simply put, anodizing is a process that changes an aluminum surface to aluminum oxide. It increases wear resistance, and the various types (e.g. hardcoat anodizing) have additional benefits like corrosion resistance.

The Process

Before the actual anodization process can start the alloys must be cleaned using specific methods. The anodization process begins by passing a direct electric current through an electrolytic (often acidic) solution in which the aluminum serves as the anode. The current creates a build-up of aluminum oxide, which the acid dissolves. The acid solution works with the oxidation rate to form a porous coating, and those pores allow the current to reach the substrate and further increase the coating.

Nimet uses traditional and proprietary processes that yield excellent results

Nimet offers both proprietary and traditional finishing processes. More detailed information is available elsewhere on our website, and we thought a more general explanation would be helpful, too.

Traditional Processes

Hardcoat Anodizing: Produces an oxide coating that is denser and thicker than clear anodize.

• Benefits: Wear resistance. Insulates components. Corrosion resistance.
• Sample Applications: Usually applied to machine-wrought aluminum.

Clear or Color Anodizing: Produces a nearly clear oxide coating which is usually less than 0.0010″. thick. Typically we seal the coating to improve corrosion protection and color-fastness.

• Benefits: Corrosion protection in mild environments. Some wear resistance.
• Sample Applications: Used in many engineered aluminum components like windows and door frames.

Proprietary Finishes

Nituff®: Nimet’s proprietary anodizing process that transforms the properties of the treated metal, which increases its hardness. Nituff yields a product that is highly resistant to corrosion and has a low coefficient of friction, a clean and dry-lubricating surface, and strong dielectric properties.

• Benefits: Light weight, easy machinability, and low cost.
• Main Properties: Extremely hard, self-lubricating, corrosion resistant, reduces friction
• Sample Applications: Enhances highly critical components in aircraft and aerospace applications, Decrease clean-up time or improve the release characteristics of injection molds, hot melt glue equipment, spray paint applicators, and so on; Provide easily cleaned surface for medical, dental, pharmaceutical, and food processing equipment that are approved by the FDA.

Black Nituff®: Nimet’s proprietary Nituff anodizing process that includes a rich, black finish.

• Benefits:  Light weight, easy machinability, and low cost.
• Main Properties: Corrosion resistant, self-lubricating, aesthetically appealing

Nituff C®: Nimet’s proprietary Nituff anodizing process that includes clear finish.

• Benefits:  Light weight, easy machinability, and low cost.  More resistant to corrosion and abrasion than standard color anodize.
• Main Properties: Extremely hard, self-lubricating, corrosion resistant, reduces friction

Anografics®: Nimet embeds customized photorealistic four-color images on aluminum surfaces.

• Benefits:  Wear and corrosion resistance – neither the image nor the color will flake, peel, or rub off.
• Main Properties: Resistant to scratches, abrasion, solvents, and chemicals.
• Sample Applications: Outdoor signage

Learn more about our processes.