Figure 1: Insert Assembly in Aluminum
The KATO Tech Group has recently received inquiries as to whether or not the tried and true practice of coating tapped holes in aluminum with zinc chromate or epoxy primer can be eliminated, by coating the inserts with a fluoropolymer or similar coating. The purpose being to prevent galvanic corrosion in the dissimilar metals; in this case the stainless steel inserts and the aluminum parent material.
Our immediate response was no, coating the inserts is not a replacement for coating the tapped holes with zinc chromate or epoxy primers per Federal Specification TT-P-1757, MIL-P-24441, or equivalents. However, we researched this subject and sought the opinions of experts in the field, including a fastener engineer, a corrosion engineer, and a metallurgist.
First, a little background. Zinc chromate primer has been, for as long as we can determine, the process of choice of the military and aerospace communities, for preventing or minimizing galvanic corrosion, specifically in bolted assemblies utilizing stainless steel helical coil inserts installed in aluminum tapped holes. Typically the primer is applied to the tapped hole in a thinned condition and very sparingly. The insert is then installed while the primer is still wet. The primer forms a barrier between the aluminum parent material and the stainless steel insert to prevent galvanic corrosion.
Galvanic corrosion occurs when metals of different electrical potential are in contact in the presence of an electrolyte . A low energy current flows from the metal having the higher position on the galvanic series (in this case the tapped threads in the aluminum parent material) to the metal in the lower galvanic series position. The aluminum or anode would proceed to corrode, while the stainless steel insert or cathode would remain relatively unaffected, or would corrode slower than it would if alone.
Figure 2: Detail A
The concern that we have is that by leaving the aluminum parent material unprotected, particularly in the presence of a salt water electrolyte, an electrical cell will be set up between the exposed aluminum, and the uncoated stainless steel bolt, allowing current to flow, resulting in corrosion to the more anodic material; in this case, the aluminum tapped threads. For reliability and safety, design engineers usually design the fastener (for example the bolt or screw) to fail before the primary structure. This concept should definitely extend to corrosion considerations, i.e. that the critical (and usually expensive) aluminum housing not corrode before the fasteners! Obviously it is very quick, easy and inexpensive to replace a corroded bolt vs. the aluminum part.
Figure 1 shows the anodic areas of the tapped threads in aluminum that are not coated with zinc chromate primer, and thus would be subject to corrosion.
These areas include the tapped threads, the countersink, the partial (chamfered) threads at the bottom of the hole , and the floor of the blind hole. Detail A gives a magnified perspective of the unprotected areas relative to the cross-section of the insert. If the aluminum tapped threads were coated with zinc chromate primer, the chromates in the primer would prevent or minimize corrosion by limiting the current density in the electrolyte, thus protecting the aluminum.
Figure 3: Detail B
Direct Attack Corrosion: This type of corrosion is usually a simple chemical attack. The level of attack is dependent on the amount of water and salt present. If the tapped threads are not coated with zinc chromate primer, corrosion or oxidation of the uncoated aluminum is a concern, particularly because the corrosive effects are not visible and can affect the structural integrity of the area under attack. The zinc chromate coating, although not as resistant to attack as an anodic finish, would be significantly more corrosion resistant than if the aluminum were uncoated.
Detail B gives an excellent perspective of the unprotected aluminum in the area of the countersink, including the proximity of the stainless steel bolt and mating part. This area would also be subject to direct attack if not coated with primer. If proper precautions were not taken to seal the assembly at the top, the degree of corrosion would be amplified depending on the type and amount of electrolyte or corrosive that enters the tapped hole.
After thorough research and review, the KATO Fastening Systems’ Engineering Department concludes that eliminating the coating of the aluminum parent material threads with zinc chromate or epoxy primer makes the assembly vulnerable to both galvanic and direct attack corrosion, particularly in the presence of a salt water electrolyte. Moreover, using inserts coated with a resin-bonded fluoropolymer coating is not a replacement for the tried and true practice of applying zinc chromate or epoxy primers to the tapped holes.