What metal is usually employed as a sacrificial anode for corrosion control purposes?

Zinc bar

The goal is to stop rusting (oxidation) by using zinc to sacrifice, since it can hold a greater "negative electrochemical" potential.

It is the difference of the ability to hold the negative electrochemical potential between the zinc and metal to be protected that causes the "sacrificial" metal (zinc) to corrode.

Zinc is useful in salt water for ship hulls.

For more info, please see the American Galvanizers Association site for article on Sacrificial Anodes

Also, see CorrosionPedia site for article on Sacrificial Anode

And for detailed info, please see ScienceDirect site for the article on Sacrificial Anode

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What is the relative dielectric constant for air?

1

From mattd2414:

Dielectric constant, also called relative permittivity or specific inductive capacity, property of an electrical insulating material (a dielectric) equal

• to the ratio of the capacitance of a capacitor filled with some MATERIAL

• to the capacitance of an identical capacitor in a VACUUM without the dielectric material.

If C is the value of the capacitance of a capacitor filled with a given dielectric

and C0 is the capacitance of an identical capacitor in a vacuum,

the dielectric constant, symbolized by the Greek letter kappa, κ, is simply expressed as κ = C/C0.

The value of the static dielectric constant of any material is always greater than one, its value for a vacuum.

The value of the dielectric constant at room temperature (25 °C, or 77 °F) is 1.00059 for air.

Because the value of the dielectric constant for air is nearly the same as that for a vacuum, for all practical purposes air does not increase the capacitance of a capacitor.

For more info, please see Encyclopedia Britannica's article on dielectric constant

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Which metal object may be least affected by galvanic corrosion when submerged in seawater?

Stainless steel propeller shaft.

Stainless steel does not corrode, as neither does copper. That is why moonshine is made in either stainless steel or copper stills, the only two materials that will not even react to alcohol.

Salt water won't affect stainless steel propeller shafts either.

For more info, see Wikipedia's article on Stainless steel

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Skin effect is the phenomenon where:

RF current flows in a thin layer of the conductor, closer to the surface, as frequency increases.

For more info, please see Wikipedia's article on
Skin effect

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Corrosion resulting from electric current flow between dissimilar metals is called:

Galvanic corrosion.

The question is the definition of the answer.

For relevant and brief explanation, please see the American Galvanizers Association site's article on Galvanic Corrosion.

For more info, please see Wikipedia's article on Galvanic corrosion

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Which of these will be most useful for insulation at UHF frequencies?

Mica.

Please see Wikipedia's article on Mica, particularly on use in Electrical and electronic

For more details, please see National Library of Medicine site article on Mica/Epoxy-Composites in the Electrical Industry: Applications, Composites for Insulation, and Investigations on Failure Mechanisms for Prospective Optimizations

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