The necesity of considering cavitation erosion a mechanic phenomena, against chemical corosion.

Bordeasu, Ilare ; Anton, Liviu Eugen ; Baya, Alexandru 等

1. INTRODUCTION

In last 30 years (Franc & Michel 2004, Mitelea et al. 2005), cavitations erosion is considered, by specialists, a typical local fatigue phenomena of the material, which suffered deformations or/and breakings under repeating action of micro jets and shock waves generated by cavitation bubbles implosion. Scientific researches, done in special cavitation laboratories, shows that damage level produced by cavitation erosion depends, both by cavitaion intensity, specific to hydrodynamics of the cavitating flow (which includes nature and physical properties of the liquid) and, by the nature of the material (Bordeasu 1997). Because of these, large theoretical and experimental studies are realized in order to correlate resistance to cavitation erosion with mechanical characteristics of the material and with hydrodynamic parameters of cavitaion phenomenon (Garcia et al. 1967, Bordeasu 1997, Franc &Michel 2004).

In the paper there are presented a number of caviattion erosion results tests, made in various liquids, and using materials with different cavitation erosion resistance, in order to reveal mechanical aspects of cavitation erosion and to motivate the cavitation process approach by hydraulic specialists, against general corrosion. To reach this goal, mass loss at various cavitation regimes is used like analyze parameter. In the paper are used only results of cavitation erosion produced in vibratory apparatus, because of the high intensity of damages process.

2. MECHANICAL ASPECT OF CAVITATION EROSION

If cavitation erosion was observed long time ago, the firs technical cases (ship propellers and hydraulic turbine runners) eroded by cavitation phenomena, were noticed about 1890 (Bordeasu 1997). General aspect of the erosion areas, present following characteristics:

* cavitation erosion of the material is selective;

* individual damages have the aspect of roughness grooves, often parallel disposed ;

The opinion that cavitation erosion is mostly a mechanic phenomena is sustained also by following experimental results (Franc & Michel 2004):

a) it have been obtained rapid erosions in situations in which in non cavitating flow (monophasic flow), for the same couple of liquid--solid, chemical reaction were unrelevant (for example oil product/glass, mercury/plexiglas);

b) microscopic observation of some caverns produced in few imploded bubble areas leads to the conclusion that the caverns have the same configuration to moon's craters, that is symmetric holes with often singular border ring

3. EXPERIMENTAL FACILITY

The paper results are obtained on cavitation eroded materials in vibratory apparatus characterized by the highest damaging intensity (Bordeasu et al. 2007).

It have been analyzed cavitation eroded materials in vibratory apparatus from Hydraulic Machinery Laboratory of Timisoara (named THML) (Bordeasu et al.2007) and from Michigan University Laboratory (named MUL) (considered the best one)) (Bordeasu 1997, Garcia et al. 1967).

Operating parameters of Michigan (Garcia et al. 1967)

vibratory apparatus, which results have been used in present paper considerations, are as follows:

* Power consumption 200W

* vibrations aplitude 12.5 -100 [micro]m

* vibrations frequency = 20000 Hz

* probe diameter = 14.3mm

4. RESULTS AND DISCUTIONS

For a pertinent analyze in order to justify the necessity of approaching cavitation erosion as a mechanical process, in figure1 and 2 were shown Bordeasu results on various materials probes tested in TMHL on magnetostrictive vibratory apparatus with nickel tube (Bordeasu et al.2007), also in various liquids. For the same goal, in figure 4 are presented (Garcia et al. 1967 results, and in figure 3 are presented Sisak (Sisak et al. 1980) results.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Note: DW--distillated water; TW--drinking water; MW-standard sea water; Em--emulsion; Ra-average roughness of probe surface before cavitation erosion. Here, Ra=0,2 [micro]m.

[FIGURE 3 OMITTED]

Note: DW--distillated water; H2SO4--[H.sub.2] S[O.sub.4] ; (NH4)2SO4-[([N.sub.H4]).sub.2] S[O.sub.4]; 10%, 20% 30%--substance percentage in water).

[FIGURE 4 OMITTED]

Note: DW--distillated water at 21C; Hg -mercury at 21C;PbBi-lead--bismuth at 260 C; 304 and 316 stainless steel; OLC--carbon steel.

The results presented in fig. 1 and 2, show for the same material (alloyed steel 40Cr10, bronze CuAl III RNR) independent by working fluid (distillated water, drinking water and sea water), mass losses are produced only by mechanical effect of cavitation erosion, without substantial contribution of surface or intergranular corrosion.

Cavitation liquids and probes were chosen so, to reveal both cavitation erosion independence by chemical corrosion, through effect of upper mentioned factors on cavitation damages, as well as the plurality of cavitation and chemical corrosion effect.

These mass losses are mostly the same, the differences between them are included in deviation limits according to ASTM standard, under [+ or -]15%, specific to complx phenomena, such as cavitation erosion.

Datas showh in fig.3,4 reveal the effect of substance percentage in chemical aggressively solution upon material damage during cavitation erosion..

4. CONCLUSIONS

1. It have been demonstrated why cavitation erosion must be considered like a mechanical process of materials damaging and a distinct part of corrosion. In ordre to reach this goalit have been analyzed the results of cavitation erosion tests upon various metals tested on vibratory apparatus in TMHL and MUL. Basic of mechanical aspect were done by cavitation erosion tests performed in distillated water drinking water standard sea water and emulsion

2. It was clarified the effect of cumulated action of chemical corrosion--cavitation erosion upon damage level of materials during cavitation phenomenon. It was possible because various liquids chemical aggressively were used.

ACKNOWLEDGMENTS

The present work has been supported from the National University Research Council Grant (CNCSIS) PNII, ID 34/77/2007 (Models Development for the Evaluation of Materials Behavior to Cavitation)

5. REFERENCES

Bordeasu, I. (1997). Eroziunea cavitafionald asupra materialelor utilizate in construcfia masinilor hidraulice si elicelor navale. Efecte de scard, Teza de doctorat, Timisoara. (Cavitational erosion upon materials used in manufacturing hydraulic machinery and ship propellers. Doctoral degree thesis)

Bordeasu, I., Popoviciu, M.O, Mitelea, I., Anton, L.E., Bayer, M., Funar, S.P., (2007).. Cavitation Eroded Zones Analysis For G-X 5CrNi13.4 Stainless Steel Proceedings of The 18th International DAAAM Symposium "Intelligent Manufacturing & Automation: Focus on Creativity, Responsibility and Ethics of Engineers", Zadar Croatia, 24-27.10.2007, pp.105-106, ISSN 1726-9679, ISBN 3901509-58-5

Franc, J., P., Michel, J., M. (2004). Fundamentals of cavitation, Kluwer Academic Publishers-Dordrecht/Boston/London, 2004, ISBN 1-4020-2232-8 (HB)

Garcia, R., Hammitt, F.G., Nystrom R.E. (1967). Correlation of Cavitation Damage, with Other Material and Fluid Properties, Erosion by Cavitation or Impingement, ASTM STP 408, American Society Testing Materials, p.239-279

Mitelea I., Bordeasu I., Hadar A., (2005). The Effect of Nickel Content Upon Cavitation Erosion for Stainless Steels with 13% Chromium and less than 0,1% Carbon, Chem.Abs. RCBUAU 56(11) Revista de chimie Vol.56 Nr.11, vol.56, pp.1169-1174, ISSN:0034-7752,

Sisak A, Kuzman F.F., Kuzman-Anton R., Potencz I. (1980). Eroziunea cavitafionald in solufii apoase de diferite concentrafii ale unor substance chimice, Memoriile sectiilor Stiinifice ale Academiei RSR, Seria IV, Tomul III, nr.2, Iasi, 1980, p.197-221, ISSN 1224-6077.

*** Standard method of vibratory cavitation erosion test, ASTM, Standard G32-8