Scott Sherer's Recent Article on Exterior Screw Replacement
Scott Sherer
COO Forum Moderator
I have just received a "letter to the editor" about my recent article on exterior screw replacement. I'm interested in your comments to see if I got this wrong. Here's the letter(s).
"Wake Up Scott!
Screw corrosion may not be a concern for you now, but by replacing the the original steel screws with stainless steel hardware, as you described, you have increased the galvanic corrosion potential of every fastener replaced. Currently the most active element in the aluminum/stainless junction is the aluminum and it will sacrifice itself and corrode to “protect” the new screws.
Cessna knew about galvanic corrosion so when they installed steel screws in their production they used cadmium plated hardware. This plating provides a level of protection for the fastener against rust and more importantly, as it is more active than aluminum, the plating sacrifices itself to protect the adjacent aluminum from the steel fastener’s corrosive affect. Airframe screw hardware typically creates inconvenient corrosion. It is crucial, however, that structural hardware be replaced with its cad plated original style fasteners or with other specifically approved replacement fasteners.
Some seaplane maintenance shops have started using a relatively new product that keeps the electrolyte away from the junction of the dissimilar metals. Experience has indicated that using “Tefgel” stops or at least dramatically reduces the stainless steel fastener corrosion common to GA aircraft. Cad plated hardware is superior to stainless when it is prepared properly and replaced at intervals well prior to the “brown stain”. Stainless is a good alternative when used with a performing corrosion inhibitor and fastener areas are monitored for corrosion.
Harry Shannon
Amphibians Plus
Bartow, Florida"
Scott Sherer
Wright Brothers Master Pilot, FAA Commercial Pilot
Aviation Director, Cessna Owners Organization Forum Moderator and Cessna Owners Author.
Need help? Let me know!
Comments
The original cad plated 1978 vintage screws on my Cessna fuel cap mounting plate were well corroded so I replaced them with stainless fasteners. With original screws removed carefully take a scotch bright pad in a twisting motion to remove surface corrosion on the countersunk flare of the hole. I then wiped or sprayed LPS-2 lube on the flared surface of the plate and dipped the screw threads and head in LPS-2 as well. The lube film is intended to separate the metals to stop corrosion like the Tefgel described above. It’s been a year so will take a few screws out and post photos for review. We cover the Corrosion topic in greater depth in the Paul New seminar as well.
Scott Sellers, COO Editorial Board
Roger that, Scott. Much thanks for your comments and I'm very much looking forward to the seminar!
Scott Sherer
Wright Brothers Master Pilot, FAA Commercial Pilot
Aviation Director, Cessna Owners Organization Forum Moderator and Cessna Owners Author.
Need help? Let me know!
The subject 'Can Stainless Steel & Aluminum Be Used Together' is covered by Marsh Fasteners at https://www.marshfasteners.com/can-stainless-steel-and-aluminum-be-used-together/ where they say: "Although aluminum reacts negatively to stainless steel, large surface areas of aluminum to stainless steel can be acceptable depending on local environmental conditions. Severe corrosion is likely to take place in a marine environment. However, there are methods that can be used to reduce this effect. A good way to reduce corrosion is to use an isolating coating or paint on the aluminum and the steel to isolate them electrically. "
After observing stainless steel screws in Cessnas for at least 30 years I've yet to seen an airplane where corrosion was a problem due to the stainless fasteners. By lubricating the screws while installing potential issues are averted due to isolating the metals. Here's a slide from the Paul New corrosion course showing the fuel cap panel on my 182RG.
The original cadmium plated screws are well corroded after 40 plus years. A year after installing stainless screws with LPS-2 here's the same panel.
You can find the Paul New Seminar on corrosion here: https://cessnaowner.org/common-sense-maintenance-issues/ This content is the most comprehensive examination of corrosion in GA aircraft I've ever seen. Any questions or comments please let me know. Thanks