Capacitance measurements as a function of time can give specific information on the uptake of water

by paint Nms. More recently it has bcm shown that information on dielectric behadour of paint before

and after immersion in corrosive medium can be obtained by the measurement of capacitance as a

function of frequency. The resistance and capacitance values of different indigenously prepared paint

schemes were compared with imported schemes. The results are discussad h relation to the water u p

take of the paint fdms

Guruviah, S.

Jayakrishnan, P.

IR@CECRI

Bulkin of Electrocherniatry Vel.2 (4) July-Augurt I=, pp.371-373 EVALUATlON OF PRQTffieTIVE SCHEMES BY CAPACITANCE AND RESISTANCE MEASUREMENTS P JAYAKRISHNAN and S GURUVIAH Contrd Electrochemical Rawarch Institute, Karaikudi - 623 006 ABSTRACT Capacitance measurements as a function of time can give specific information on the uptake of water by paint Nms. More recently it has bcm shown that information on dielectric behadour of paint before and after immersion in corrosive medium can be obtained by the measurement of capacitance as a function of frequency. The resistance and capacitance values of different indigenously prepared paint schemes were compared with imported schemes. The results are discussad h relation to the water u p  take of the paint fdms. Key Words: capaci t .nce1rsrkt .n  meuumment,  mint w h o m r  protoctlvo coating INTRODUCTION t has already been established that changer in electrid r9fii5-e~ e t a  paint f h  in a medium cm be r&@d to its ma ~ v l o u r  I]. Both DC and AC meth-, g~ \IKd & W M ~  0 1 d C . l  resistance '2.d. The par? 19 tqc: paint fw @ye w to tho oonodw medium to ittack the bfiq nM y d  hence the conlpruativo protoctlve property can assessed from the change in in the resistance. Capacitance of the paint T i  can provide some information on the absorption of water by tho m, It has further beqn reported that measurement of t h w  p r o p  tie$ at different frequencies can piye p ~ r ~  j a m t t g l l  M th khybw 9C the paipt fJTtl5.. 10 oms r lo cms mild steel panels were sand blasted, d e g r d  and were ~ o a d  M tho cubstrato material. The following paint schemes, prepared in- rligenouely were applled: 1. Epoxy polyamidc undercoat . . SO p 2. Zinc rich primer + undercoat .. 88 p 3. Zinc rich primer + undercoat ., 12Sp 4. Undmwt  + Ankh coat .. 123 p 3, Zino rlch prlmor + undernut + finish coat .. 223 p Fw paarRarlson purpo#o, the s ~ l o  paint schemes with imported fonnula- ~wsrcabocoatedmchanesaumbored la.2a.h,4a,andSa).Resistana a d  capldtance measurement8 ware carried out in two media, 3% sodium chlaride and 5% sodium hydroxide, usinp the universal bridge B 224 (Wayne Kerr, UK). Two froqumcies of 1 KHz and 10 KHz were chosen for which an external w u r a  was used. Glass t u b  of 1 cm dl. and 4 cm were fued o w  tho coating surface to facilitate holding of the solution and p l a t i n u ~ ~  wan wd as the I d .  The studies were carried out over a period of 30 day. l k t r ~ m o t ~ ~ ~ p t l k e ~ f t k f l k  a h  fits of oils 5.0 am x 7.3 cme were cleaned well, weighed and each of the plat achema m~lltloaedabove was applied o v a  the aame and cured and d & e d  again. Tho diffaeaa gives the wdght of the paint costal on g h .  The coated phtm w a e  immersed separately in distilled water. The pmols nsro removed periodically and the surface water was cleared with the hdp of fdter papa and wdghcd again. The difference gives the amount of wata absorbed. From this the pucentage of wata uptake is calcuhed. RESULTS AND DISCUSSIONS Tables I to IV give the variation of resistance and capacitance of different paint schemes, in sodium chloride and sodium hydroxide solutions, at dif- ferent frequendes in a period of 30 days. Table I: The change in reJltancelcapecitance of the painted panela with time in 3% NaCl at 1 KHz Paint Schane Resistance (Mcgaohms) Capacitance (Pq No. Initial Final Initial Final JayakrWlnan end Guruvieh - Evaluation of protectiva w m e s  by capacitance and resistance msa8uremmta Tafh  It: The change in redsten&/capadtance of the painted panels in 3% NaCl at 10 KHz Scheme Rc&mce ( M e g a o ~ )  Capacitance (PF) No. Initid F i f l  i~ljtial Finar Tabla Ill: The change in restrtenw/capacitance of thr printed panels in 6% NmOH at 1 KHz Scheme Resistance (Megaohme) Capacitance (PF) No. Initial Final Initial Final Tabla IV: The change in ~,nce/capacttance of the painted paneb in 6% M H  a 10 KHz Scheme Resistance (Megaohme) Capacitance (PF) No. Initial Initial Final Fie. 1 : Chonge in cupocitonce of thepqintedpgne/s yith time in 3% Noel at 1 KHz It can be seen that the best performance is obtained with paint scheme No. 5 on the expected lines- because of the muboats given and also of the h@a thickness of the multiwat. But in the case of the coathi! with Zinc rich primer and undercoat, the initial capadtana value itself is high and ac- cordin& the resistance bao shown a downward trend. This is well m- plimented by the rcsuJta obtained by tkc water uptake experiplents which is shown in Fig. 2. Fig 2: Percentage woter uptoke of the prrinted prrneb with time 372 Bulktin of V d 2  (4) July-August 1988 Jayakrishnan and Guruviah - Evaluation of protective scheme9 by tapaeitance and resistance meaaurementa I n  systems 2 and Za, the trend is that there is maximum uptake of water CONCLUSION d u d q  the atid stage and afterwords the saturation point is reached. However, when the thickness of the llndercoaf is itlcwnsed, this trend is By comparing the coatings that have baen formulaad with those of the reversed, so i t  could be that the undercoat should be highly non-porous imported samples, i t  is quite clear that the indigenous samples possess almost for getting best results. This bchaviour may be due to the high zinc con- identical properties to imported ones, as far as the behaviour in the media tent of the mating. However, the law resistance cannot be attributed to studied are concerned. The behaviour i s  almost identical at different fre- the low corrosion resistance of the film as the mechanism of lction of zinc quencies also. rich primer is entirely different. REFERENCES The multicoat with zinc rich primer, undercoat and fhsh coat is atrerndy 1, s GUN* and K s Rajagopalan "Com/ation of Physic0 chemkalpm corrosion resistant and practically there i s  not much change in  the p~fities and syuctural C ~ ~ F U C ~ ~ F & ~ ~ C S  with mint performances" January I capacitance or resistance values, when they are put under test for 30 days. 1977, National Test House, Calcutta From the water uptake also i t  is clear that there i s  not much change in  the water absorptioh values even during the initial stages. Water uptake after 2. Kittlebcrger and A C Eh.* Ind Eng 44, 326 five days is practically negligible, which can be very well Jean from the 3. JEO M ~ ~ ~ ~ ,  J oi l  col them A ~ ~ ~ ~ ,  40, (1957) 183 variatioh of capacitance brith time (Figs. 1 and 2). In  almost all system, the capacitance value increases in the first few days and shows a steady 4. S Guruviah and P Jayakrishnan, Paper presented in  the III Int Svmp trend indicating a saturation in the water uptake of the films. Adv Electrochem Sc and Tech, Dec. 1984 /r" ALUMINIUM LITHOPLATES Proceas developed at Central Electrochemical Research Institute, Karaikudi 623 006, Tamil Nadu, India. (Indian Patent No. 98606) Aluminium lithoplates are used in offset lithography, photo offset lithography and multicolour printing of labels, calenders etc. and in all fields where the zinc plate8 ate u d .  Further, in rotaprinting and duplicating machines where thick gauge aluminium (0.127 mm) are used. The platet procemwd by this method will substiitute the im- ported aluminium anodized plates. It is estimated that by setting up indigenous production, there will be foreign exchange saving to the extenl of Rs.3 million. I Proceu developed I The new proms is beJed on ttie princide that the surface of an aluminium plate gets etched when treeted chemical- . . ly or eiec&chemkally in solution containing alkali &, chromatea, phoshate9, fluorides or chloride ions at suitable concentrations producing a uniformly grained surface. This is subsequently oxidiied anodically in the electrolyte containing chromic acid, sulphuric acid andhr phosphoric acid. The process offers the following advantages: 1. Use of zinc which ,is an imported material has been eliminated. 2. The surfce of aluminium offset lithoplates is smoothly and uniformly grained to a controlled size and depth producing a porous structure. 3. When mechanically grained aluminium plates are used they developed a particular type of 'scum' known as 'inkdot scum' during printing. The present process has obviated this importantdiidvantage by a subsequent anodizing step after graining. 4. The wear resistance and corrosion resistance are also improved. 5. The grained and used plates can be regrained and reused, if necessary. 6. The process is abm suitable for graining and anodizing thin (0.1 mm) once-used aluminium plates for d u p l i i n g  machines etc. I The process has been cornmercialii to manufacture plates of maximum size 135 cm. x 120 cm. x 0.0825 cm. I Lithographic aluminium plates prepared in the laboratory were put to printing trials at Coronation Litho Works, Sivakasi. The q u a l i  is reported as good as the imported sample and better than conventional zinc plates in every respect, and are now in regular use in some presses in this country. A# the raw materiels such es chromic acid, hydrochloric acid, d i m  hydroxide, aluminium sheet are i n d i i  availebb. Plent and machinery required are rectiftr, transfomr, air compressors. exhaust blowers - all availeble in the country. I I Bulletin of - i  Vd.2 (4) July-A- 1988 373 

Evaluation of protective schemes by capacitance and resistance measurements

Abstract

Similar works

Full text

Available Versions

IR@CECRI