US2230273A - Preventing erosion of metals - Google Patents

Description

Feb. 4, 1941,.

Filed No. s, 19m

coaed wif/7 ame/v//c soap.

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Patentedv Feb.4 4,` y1.9.41 .v

I PATENT OFFICE PaEvEN'rING EnosIoN oFfME'rALs Edward W. Smith, Melrose, Mass., assignor to Submarine Signal Company, Boston, Mass., a

i corporation of Maine Application November 9 claims.

The present invention relates to a method for the protection of metals and more particularly to a method for preventing the .erosion of metals which are in contact with liquids in which cavitation occurs.

For some time it has been observed that the surface of diaphragme of submarine signaling apparatus which are vibrating at relatively large amplitudes become pitted or leroded after being in use for some time. the diaphragms of acoustic apparatus' used in the treatment of liquids for the destruction of bacteria or the production of dispersions and the like. Similar erosion also is found on ships propeller blades, as well as in conduits .for liquids flowing at highveloclties. It is believed 'that most of the investigators in this field are agreed that-the cause of lthis erosion of the metal surfaces in some way involves cavitation of the liquid.

In the accompanying drawing Fig. l represents in a transverse section a submarine signaling device having a vibrating diapragm in accordance with .the present invention, andFlg. 2 represents a perspective lview of'a ships propeller in accordance with the present invention.

According to the present invention theV erosion produced by cavitation is eliminated by providing the metal vwith a surface which is not wetted by the liquid in contact with which-it is to work.

This may be accomplished either b y choosing a proper metal or by coating the metal with a substance which is not wetted by the liquid.

The -mechanism whereby the erosion in question is brought about under the inuence of by the details of the action suggested below, my

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theory of the action of cavitation is as follows. Jin-examination of diaphragme which have been badly eroded by cavitation indicatesl that the metal was forcibly torn'out from the surface of the diaphragm. lIf this is true,` it is necessary that tremendous. forces be applied in order to produce this eflect,ffor these forces must be capable of overcoming the cohesive forces existing be'- tween the molecules of the metal.

It-is a'well-known fact that the surface tension of water, for instance, at ordinary room temperatures, is approximately 73 dynes per centi- The same is also .true of' s, 1997, serial No. 173,269'

meter. The work done in breaking a column of liquid vso as to result in the formation of anew surface will be equal to the surface energy Vresident in the new surfaces so formed. If it be as;- sumed that separations of the order of molecular distance, that is, 10-a power centimeters, are to' be brought about, it can readily'be Ishown that a. force of approximately 16,500-pounds will be required to bring about a cleavage in a Water column of one square centimeter in cross section. l0 Forces of this order of magnitude are. great enough to bring about al removal of molecules' 'from a metal surface if` they are properly applied.

Moreover, it appears that the surface tension of a liquid at the boundary between the liquid and a metal or other material is tied up with the ability of the molecules of the liquid-to diffuse into the metal or other surface material. As stated above, the surface tension ofwater in contact with air is approximately .'73 dynes per c m. Whereas the surface tension of water in contact with mercury is approximately 392 dynes per cm. and in contact with water the surface tension of Water is zero. Itis reasonable to conclude from these figures that water or other liquid has very l little afnity for la substance, contact with which produces a high surface tension in the. water and, furthermore, that the water molecules can diffuse into such substances only with the greatest difficulty.

On the other hand, when in contact with substances which produce a low `surface tension in the water, the water molecules can diffuse much more readily 'into the surface of such substances. Stated in another way, it is generally true that the surface 'tension of the liquid will be inversely proportioned to the ease with which it fwets e the material with which it is in contact.

Let us` assume, now, that'we have water in contact with a metal such as, forexample, a vibratory diaphragm as shown in'Fig. l and which is vibrated at cavitational velocity by the action of electromagnet '2 upon the armature 3; or a propeller blade as shown at. t in Fig. 2; or a conduit through which-water is flowing at a high velocity. It will be apparent that some of the water molecules can migrate or diffuse into the surface of the metal ofthe .diaphragm and form a bond'therewith. If the surface tension of the water in contact with such a metal is relatively low,1the diffusion of the-water molecules into the metal will be relatively high. In other words, the water will wet the 'diaphragm` If such a diaphragm which 'is wetted by wateris 55 then set into-motion at cavitational amplitudes, the bonding between the liquid and the diaphragm which results from the diffusion of the liquid into the diaphragm can only be broken by high diaphragm velocities such as are obtained when cavitational amplitudes are reached.

When the diaphragm pulls away from the water at such a velocity, forces of the magnitude above referred `to will be applied. The cohesive forces between the metal molecules are, however, of

the same order of magnitude and may even be slightly less than the cohesive forces between the Water molecules. Consequently, particles of the metal may pull away from the mass of the dia.-

phrag'm itself. Now, ii the material of the diaphragm were made of such a substance that the water molecules could not diffuse into it or, in other words, if the water did not wet the diaphragm, the attainment of cavitational ampli# tudes would not be accompanied by disruptive action in the diaphragm, but a separation, if any,

p would take place at the diaphragm-water interface or in the water. According to the present invention a vibratin diaphragm or a propeller blade or other body subjected to relative motion in contact with`a liquid at cavitational velocity is made of a substance which will not be wetted by the liquid with which it is in contact. Stated in another way, the body in question is made of a material which produces at the interface a high surface tension in theliquid which is in contact with it. As an example, a vibratory diaphragm or a propeller blade or a water conduitmay be made of self-lubricated bearing brass or bronze, which is not appreciably wetted by water. This material is a brassv or bronze containing graphite, the surface tension of liquids in contact with it being greater, the great-l erthe amount of graphite.

Inasmuch as the entirephenomena of erosion by cavitation, as above discussed, resides in the surface of the material in contact with the liquid,

' it is not necessary that the entire metal be made of-a substance which will not be wetted by I the liquid, provided a surface can be formed onY the metal which has the properties above referred to. As an example of this, a bronze diaphragm can be protected by treating the 'surface with mercury so as to form an amalgam and thus to present a mercury surface to ,the'water with which it is to be in contact. Since the mercury p that they have a relatively high aillnlty .for the metal. For example, some of the metallic soaps used for high pressure lubricants have an affinity for metals such as bronze .or steel. In this case the metallic end of the soap molecule becomes afi fixed to the diaphragm and leaves the long chain hydrocarbon end of the molecule in contact with the liquid. Since the latter is not wetted by the liquid, such as water, to any appreciable extent, there will be no erosion of the diaphragm;

From the above discussionv and the illustrative l examples given, it will be -reafdily understood by those skilled in the art thataccording to my invention it is merely necessary, in orderto prevent the erosion of a body by cavitation ,to choose a substance whose surface willbe wetted to a mini# mum extent by the liquid which is in contact with it or, in other words, the surface-'of the substance should be chosen such that the surface tension of the liquid in contact with it is as high as possible.

Having now described my invention, I claim:

l. A body having a surface adapted to be submerged in and moved relatively to a liquid with a velocity large enough to produce cavitation, said bodyhavling its said surface formed of a material such that the surface tension of the said liquid in contact with said surface is very high and the material is not substantially wetted by the liquid, whereby erosion of the body by cavitational action is inhibited. j

2., A body having a surface adapted to be. submerged in and moved relatively tov a liquid with a velocity large enough to produce cavitation, said body having its said surface coated with a substance which has a high afilnity for the body but tact with a liquid with a velocity large enough to produce cavitation, saiddiaphragm having its said surfaces formed of a material such that the surface tension of the said liquid in contact with said surface is very high and the material is not substantially wetted by the liquid, whereby erosion of the diaphragm by cavitational action -ls inhibited. l

4. A vibratory diaphragm having a. surface adapted to be submerged in and vibrated in contact with a liquid with a velocity large enough to produce cavitation, said diaphragm having its said surface coated with a substance which has a high aillnitytfor the diaphragm material. but for which the liquid has substantially no ailinity,

whereby erosion of the diaphragm by cavitationall action is inhibited.

5. A vibratory diaphragm having a surface adapted to be submerged in and vibrated in contact with a liquid` with a velocity large enough to produce cavitation, said diaphragm having its energy radiating surface coated with a metallic soap.

6. A ship's propeller tated in the water with a velocity large enough to produce cavitation, said propeller blade having itssurface formed of a material such that the surface tension ofthe' water in contact with it i is. very high and the material is not substantially wetted by the water, whereby erosion of the propeller blade by cavitational action is inhibited.

7. A ships propeller blade adapted to be roblade adapted to-be rotated in the water with ,a velocity large enough to produce cavitation, said propeller blade Having its surface coated with a substance which has a high4 aflinity for the propeller blade, but forwhich water has no substantial ailinity, whereby erosion of the propeller blade by cavitational action is inhibited.

l s, A ampia propeller blade adapted to be nr-- tated inthe water. with a velocitylarge enough to l produce cavitation, said propeller blade having its body having its said surface coated Twith a metallic soap.

EDWARD W. SMIi'llH.v

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