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.