Subfame/cross member corrosion

Anything F-Type related......
Geminii
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Joined: Tue Aug 13, 2013 5:40 pm

Post by Geminii »

An interesting clip from a company dealing in ship corrosion.

""Commercial shipping

Anodes for vessels, boats and ships

Traditionally, many vessels are fitted with sacrificial anodes for hull protection. The steel of a structure is protected when sacrificial anodes of a less noble metal are attached to the structure. Two different alloys are particularly suited for doing this in a cost-efficient way: zinc alloys and aluminium alloys.""

Not sure using an F type structure to protect a steel sub member is 'cost-efficient' :)

Rotorcraft
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Joined: Sat Jan 12, 2019 10:26 pm

Post by Rotorcraft »

While the visual aspect is ugly, the proper question is does the level of corrosion impact the strength of the structural member enough to compromise its structural function? We can’t answer this without access to Jaguar’s finite element analysis but in normal industrial use we would expect 10% loss of thickness to be tolerable in most applications. 10% looks dreadful as rust expands the base metal. Spot corrosion up to 50% may still be tolerable but might require some derating. My money would be that the safety margin on these structural elements is substantial and that they are more relevant to chassis stiffness in certain torsional modes than direct structural strength, but that is guesswork. Bottom line is that Jaguar’s position on the visible corrosion is probably correct. Attempts to clean off the corrosion may expose new metal to corrosion. Painting without proper surface preparation (typically what is known as “Swedish 2.5”) requiring shot blasting is likely to trap corrosion products under the paint and probably make things worse; your wire brush is pretty hopeless really and using it on aluminium is very bad news. So the new paint job might look pretty (but who sees it) but there is every likelihood the problem will worsen under improper painting rather then get better. Bit like skin wounds, these days the docs refer to leave them open rather then smother them in bandages. The question of electrochemical corrosion as a result of the Al/Fe cell is valid but only when an electrolyte like salt water connects the dissimilar metals. Unless you are driving daily In seawater this is unlikely to be a major source of corrosion. The few hours a year in the UK on wet salty roads are unlikely to trigger much metal loss especially if the underside is well rinsed by normal rain. Claims by paint manufacturers that their product has some magic power over corrosion should be taken with a large pinch of salt; there is no magic fix, if there was Network Rail would not have spent three years shotblasting the Forth Bridge back to bare metal and using a multipack epoxy to finally stop the annual painting cycle. If painting the member makes you feel good then go for it, but I would be more interested in monitoring the member for excessive pitting and putting the problem back to Jaguar as and when the magic 10% thickness loss over the whole cross-section is being exceeded. Which I doubt it will for the majority of us not using our cars in areas with extended winter road salting.
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Tel
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Post by Tel »

Rotorcraft wrote: Sat Jan 12, 2019 11:21 pm While the visual aspect is ugly, the proper question is does the level of corrosion impact the strength of the structural member enough to compromise its structural function? We can’t answer this....

...yes we can. History.

Look back at any Jaguar and you will see that corrosion of certain models in specific areas is a common problem.

For the F-Type, underbody steel components will be the archilles heel of the model in years down the line.


Agree with the painting regimen.
I work with underwater engineering (salt water) for over 25 years and we have to protect very expensive >£4m robotic and subsea vehicles...sealing the pores of the metal is key, that's why after 4 years, my steel work is as good as the day it left the dealer.

Surprisingly if your metal is looking crappy and you cannot afford the replacement costs, you are better off smothering the metal with a thick grease than cleaning back to bare metal and respraying.

In 2014 I highlighted this corrosion issue after getting my first F type up on ramps and seeing the poor quality of the painted parts...it's taken 4 and a bit years for surface rust to appear on quite a few customers cars. As far as timelines go, depend on usage and exposure to the elements. Give them 15 years or so and the pitting will be there for a daily driver for sure.
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PhilB
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Post by PhilB »

I've had to merge a couple of topics on this subject already, but merging this one as well would just cause confusion in the main thread.
I've moved a couple of recent posts from here to the main topic: viewtopic.php?f=27&t=3085
Can you please use the main topic from now on rather than this one.
Thanks.
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