Overview of Plating Thickness Requirements

An OEM recently asked me to provide them with some basic information about plating thicknesses for their engineering group. I thought others might find the information useful...

For the metals we plate at Chem Processing, an across-the-board “standard callout” is 0.0002 – 0.0004”. If the only callout was (metal) plating, that would be the default. What follows are some specifics:


Zinc Plating:

ASTM B633, the most commonly used zinc plating spec, uses “service conditions” to delineate minimum thicknesses. This is based on the typical corrosion protection that a given thickness provides, but also by the geometry of the part. Recessed areas are going to receive less plating than outside edges and corners. We always recommend testing a design when possible.

Callout                Service Condition               Minimum Thickness
Fe/Zn 25              SC 4 (very severe)                0.001”
Fe/Zn 12              SC 3 (severe)                        0.0005”
Fe/Zn 8                SC 2 (moderate)                    0.0003”
Fe/Zn 5                SC 1 (mild)                           0.0002”

The zinc specification SAE AMS 2402 gives ranges rather than minimum thickness, and supplies expected salt spray protection for each (data is slightly different for threaded fasteners—see spec). 2402 does a good job of addressing the common issues that come up, such as geometry and “internal surface” requirements. Spec callouts alone often cannot communicate everything on a more complicated part.

Callout                Thickness Range                  Expected Salt Spray
2402                     0.0005 – 0.0007”                   200hr
2402-1                  0.0001 – 0.0003”                  100hr
2402-2                  0.0002 – 0.0004”                  150hr
2402-3                  0.0002 – 0.0005”                  168hr
2402-4                  0.0003 – 0.0006”                  185hr
2402-5                  0.0004 – 0.0007                    200hr

Chromate conversion coatings over zinc add to the corrosion protection of the plating but do not change the thickness. Zinc plating is what we call a “sacrificial coating.” It corrodes preferentially to steel (hence the white powder that forms over time on zinc—this is the zinc corroding). The chromate makes the outer layer of zinc passive and delays the corrosion reaction.


Electroless Nickel Plating:

ASTM B733, a common EN plating specification, uses “service conditions” callouts. The notes on these service conditions give examples of environments for each.

Callout                  Service Condition                Thickness
SC0                        Minimum Thickness             0.00004”
SC1                        Light Service                         0.0002”
SC2                        Mild Service                          0.0005”
SC3                        Moderate Service                  0.001”
SC4                        Severe Service                       0.003”

SAE AMS 2404 and AMS-C-26074 use Grade designations for thickness.

Grade                   Minimum Thickness
Grade A                 0.0010”
Grade B                 0.0005”
Grade C                 0.0015”

With electroless nickel, expected corrosion protection is dependent on other factors such as phosphorous content in the deposit and the post-plate thermal treatment (to modify hardness). Nickel is a “barrier coating” rather than a sacrificial coating, meaning it protects by sealing off the base material with a passive layer of nickel. Thus all other factors being held constant, a thicker deposit equates to more corrosion protection. At about 0.003”, however, it becomes difficult to avoid pitting in the deposit.


Copper Plating:

SAE AMS 2418 is divided by type.

Type                        Application                              Thickness
Type 1                     Engineering                              0.0005 – 0.0007”
Type 2                     Heat Treat Mask/Stopoff          0.002/0.0007” min


Silver Plating:

The most common silver plating specs are SAE AMS 2410, 2411, 2412, ASTM B700 and QQ-S-365. Thickness is generally per print callout.


Hard Chrome Plating:

ASTM B650 gives only two classes.

Class               Application                                                     Thickness
1                       Light wear resistance, friction reduction         0.0001 – 0.001”
2                       Repair, heavy wear resistance                        >0.001” per callout

Hard chrome is exclusively for wear resistance and lubricity. It provides no significant corrosion protection due to microcracking of the deposit. If corrosion protection is required Thin Dense Chrome can be applied (SAE AMS 2460). TDC provides wear protection but is not microcracked. It is only plated to a maximum thickness of 0.0006”. Another option is a base layer of electroless nickel for corrosion protection with chrome deposited on top of the nickel for wear. There is not an upper limit for chrome plating, but past 0.010” the deposit has to be ground back before plating can be resumed.




Additional Finishes
This is basic information about non-plated finishes.


Aluminum Anodizing:

Most anodizing is governed by MIL-A-8625, which is broken down by types. Listed thicknesses are of the aluminum oxide film. Approximately half is penetration into the base material so part dimensions change by one half listed thicknesses.

Type               Application                                                       Thickness
Type I             Chromic acid anodizing. Mostly obsolete        <0.0001”
                        due to environmental concerns.
Type II            Sulfuric acid anodizing. General wear,             0.0002 -0.0004”
                       corrosion protection. Colors.
Type III          Hardcoat anodizing. Wear protection.               0.001 – 0.003”
                       Minimal corrosion protection.


Chemfilm:

Chemfilm is similar to chromate conversion coating on zinc. It is a chemical reaction with the aluminum and does not cause appreciable dimensional change.


Stainless Steel Passivation:

Passivation is the removal of soils, oils and free iron from the surface of stainless steel to improve the inherent corrosion resistance of the material. It causes no significant dimensional change.


Feel free to ask additional questions in the comments.

Why Cadmium Plating?

Cadmium plating has a long history of use in all industries, but is becoming rare as concerns over the metal's toxicity and carcinogenicity outweigh its engineering effectiveness. Cadmium provides galvanic protection to steel, meaning that in the presence of moisture, it corrodes instead of the steel. Zinc, a much more benign metal, protects via the same mechanism, so what's so special about cadmium that it persists in critical applications such as aerospace? The corrosion products--the powdery residue that is formed as the metal corrodes--from cadmium are less significant than from zinc. Zinc's corrosion products can contribute to the premature failure of moving parts or cause galling in applications where repeated dis-assembly/reassembly is necessary (which is the case with aerospace components that are on maintenance schedules). Additionally, Cadmium has a sufficiently low coefficent of friction to facilitate repeated maintenance, and its corrosion products are relatively benign.

Cadmium is also a mainstay in the aerospace industry because of its position on the galvanic scale relative to aluminum and stainless steel. Before the introduction of composites, airframes were primarily aluminum. Where stainless steel components interface with aluminum, they are often coated with cadmium. This provides an intermediate galvanic layer the minimizes the possibility of corrosion cells forming. The industry has moved toward zinc-nickel alloy as a replacement in this application. The substitution of zinc-nickel for cadmium presents less of a challenge than replacement in corrosion protection applications, as its tribological properties are not relevant.

Cadmium is not used in space applications because of its tendency to sublimate, or transition directly from a solid to a gas, in high vacuum.

Before its health risks were recognized, cadmium was used on automobiles, retail hardware and almost every application where corrosion protection was required. Today, it is used only in a handful of specialized applications. If you are working in one of these areas where the finish is still required and need the support of a premium cadmium plating provider, contact Chem Processing.

For more information on cadmium plating, visit Chem Processing's cadmium page.

Electroless Nickel Plating

Most metal plating takes place via the application of electricity. There is a class of plating called electroless that, as the name implies, requires no electricity. The mechanism, oxidation-reduction, is the same, but it is accomplished via a chemical reaction instead of DC power. Metal ions with a positive charge are reduced to their elemental state with electrons from chemical reactions rather than electrons in the form of current. While a few metals can be deposited with this method, the most common is nickel.

Electroless nickel plating is accomplished via a heated bath (around 200F) of nickel hypophosphate. The nickel catalyzes on an activated metal surface and "grows" on the surface at a controlled rate.

The deposit is actually an alloy of nickel and phosphorous, and varying the amount of phosphorous can yield different desirable characteristics. Higher amounts of phosphorous, >10%, give the best corrosion protection. 5-10% phosphorous yields the most aesthetically pleasing finish. <5%, considered "low phos," gives the highest surface hardness. Various alloys also yield differing magnetic properties, a characteristic that the computer hard disc industry harnesses to optimize its products.

Because electroless nickel uses chemistry rather than electricity, the deposit is remarkably uniform across all geometries. This allows for internal features to be plated because there is no requirement that a surface "see" an anode (or cathode in the case of anodizing). It also facilitates predictable tolerances since there is not the variation that occurs from electricity's affinity for points and sharp edges in standard electroplating.

Where electroless nickel is used for corrosion protection, it protects by encapsulation, rather than the more familiar galvanic protection afforded by metals like zinc and cadmium. Those metals corrode preferentially to steel and sacrificially protect it to extend its service life. Nickel forms a natural oxide layer and is inert to many corrosive elements. Thus the thicker the nickel layer, the better the protection. The possibility of the coating being compromised decreases as the nickel thickness increases.

If electroless nickel is a candidate for your engineering application, or for more information on the finish, visit www.chemprocessing.com.

What Is Aluminum Anodizing

Aluminum anodizing is the process of generating a controlled oxide film on the surface of aluminum.  The purpose of this film may be to improve the appearance of the aluminum, protect the aluminum against corrosion, or reduce wear. Aluminum automatically forms a thin oxide layer in the presence of air, but this naturally occurring film does not enhance the aluminum except to slightly reduce its susceptibility to corrosion. The process of anodizing removes the natural film from the aluminum and electrochemically forms a film of known thickness and hardness. This film can be dyed different colors or have its own coloration.

The anodized film is not "deposited" but rather is "grown." It penetrates into the aluminum as well as grows outward. It is part of the metal.

The film as formed has pores that reach all the way to the base aluminum. It is because of these pores that the film can be dyed, as the dye molecules "fill" the pores. To achieve the best corrosion protection properties, the anodized film must be sealed as a final step. This is accomplished either by swelling shut or plugging the pores via thermal or chemical treatment.

The actual anodizing process is performed in a bath composed of water and mineral acids. This acids serve as electrolytes. They carry the current from the cathodes to the anode, which is the aluminum being processed.

For more information on aluminum anodizing, visit www.chemprocessing.com.

What Is Metal Plating

Metal plating is the process of electrochemically bonding one metal onto another metal to give the resultant surface certain engineering characteristics. The most common goal of metal finishing is corrosion protection. The plated metal will offer either galvanic protection, meaning it will corrode preferentially to the metal it is protecting, or encapsulation, meaning it serves as an inert barrier between the corrosive environment and the base metal. The second most common goal of metal finishing is wear protection. The plated metal is either harder than the base material or has a lower coefficient of friction, thereby preserving it from material loss, overheating, deformation or other damage caused by wear. Finally, metal plating can improve the appearance of the base material, but one or both of the aforementioned engineering characteristics (corrosion and/or wear protection) are usually sought as well.

For a full range of metal finishing services, visit www.chemprocessing.com.