Measurement of surface moisture and sulfur dioxide activity at corrosion sites

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ASTM Bulletin, 247, pp. 47-48, 1960-07-01

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Measurement of surface moisture and sulfur dioxide activity at

corrosion sites

Sereda, P. J.

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N A T I O N A L

R E S E A R C H

C O U N C I L

C A N A D A

D I V I S I O N O F B U I L D I N G R E S E A R C H

Measurement of Surface Moisture and Sulfur

Dioxide Activity at Corrosion Sites

by

P.

J. SEREDA

Reprinted from ASTM Bulletin No. 2 4 6 M a y 1960, Pp. 47-48

RESEARCH PAPER NO.

102

OF THE

DIVISION OF BUILDING RESEARCH

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Measurement of Surface Moisture and

Sulfur Dioxide Activity at Corrosion Sites

By

P.

J.

SEREDA

a steel specimen based on a day of wet- ness has been found to couclate with the measured temperature of tllc speci- nlen and the sulfur dioxide atmospheric pollution rate (I).'

Detection of surface nloisture was made possible by the development of a suitable sensing element described in earlier reports (2, 3). The amplifier suitable as a field instrument, designed by the Division of Radio and Elcctrical Engineering for use with this sensing elenlent, is described here.

Sulfur dioxide activity was measured by the lead peroxide method

(4,

which gave an average value per day of ex- posure. For correlation with corrosion rate, the value required is that corre- sponding to the activity during periods when wetness occurs on the exposed specimens, which may or may not be the same as the average value.

A

method is presented whereby this can be accomplished.

Experimental

The d-c amplifier to operate wit11 the moisture-sensing element was designed to serve the purpose for which the pH recorder was used (2,s). A satisfactory design was achieved while maintaining the necessary simplicity. The detailed circuit diagram is given in Fig. I .

The amplifier has its own calibrating circuit which feeds to the input 0 to 0.4 v, thus enabling the instrument to be preset to trigger the output relay a t the desired input voltage. This cali- brating device enables easy cllecking of the instrument in case any drift of the response occurs. The off-on differ- ential is about 0.03 v.

This amplifier has an input impedance of about 10 megohn~s, and for this reason it was necessary to use a scnsing elenlent having two platinum electrodes on cach face in order to retain the desired sensi- tivity.

Two amplifiers, each coupled with a Haydon elapsed-time indicator ED72-

O O ~ , mere used to record the hours of wetness on the skyward and ground- ward exposures from which thc time-of- wetness n-as obtained.

NOTE-DISCUSSION O F T H I S PAPER IS INVITICD, citlier for publicnt~on or for the attcntiorl of tllc author or autliors. Ad- dress nll communications to XST3I I-Icad- quarters, 1916 Itacc St., I'hilndclphia 3, Pa.

'

Tlie boldface numbers in pnreiltlleses refer to the list of references nppcnded to this paper.

This is the final report on measurement of surface moisture carried out at the Division of Building Research and the Division of Radio and Electrical Engineering, National Research Council, Canada. This project was ini- tiated by the Task Group on Measurement of Atmospheric Factors of ASTM Committee B-3 on Corrosion of Non-Ferrous Metals and Alloys.

Cannon RWK-3-31SL

I

N o t e - A l l Resistors w Unless Specified

Fig. 1.-Dew detector amplifier.

Tlle lead peroxide cylinder and screen mere modified to permit the uncovering of tlle cylinder during periods of wet- ness and to shield the cylinder during dry periods. The conlplete assembly, called the sulfur dioxide shield, is shown in Fig. 2. I t consisted of an aluminum cup v-hich was lifted by means of a re- versible, 6-v, d-c motor operating through a rack and pinion.

This apparatus mas electrically coupled with the amplifier and nloisture- sensing element in such a way that the lead peroxide cylinder was exposed only during the periods tvllen nloisture

mas detected on the groundward side of the sensing elcment. The conlplete schematic wiring diagram is shown in Kg. 3. The amplifier and clapsed-time indicator were in a heated hut; the sensing clement, the sulfur dioxide shield, and lead peroxidc cylinder were located outdoors on the exposure site where corrosion of steel mas being ob- scrvcd.

The lead peroxide cylindcr that was exposed continuously mas located be- side the one that was exposed during wetness periods so t h a t the two values could be comparccl.

P. J. S E m D A , associate research officer, Division of Build- ing Research National Research Council, Ottawa, has been engaged since 1950 in the study of the behavior of water in porous systems including methods of detecting and measur- ing the presence of water on surfaces of materials.

May 1960 (TP 107) 47

Authorized Reprint from the Copyrighted ASTM BULLETIN No. 246, May, 1960 Published by the American Society for Testing Materials, Philadelphia 3, Pa.