ASTM_G_84_-_89_2012.pdf

Designation:G8489(Reapproved 2012)Standard Practice forMeasurement of Time-of-Wetness on Surfaces Exposed toWetting Conditions as in Atmospheric Corrosion Testing1This standard is issued under the fixed designation G84;the number immediately following the designation indicates the year of originaladoption or,in the case of revision,the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This practice covers a technique for monitoring time-of-wetness(TOW)on surfaces exposed to cyclic atmosphericconditions which produce depositions of moisture.1.2 The practice is also applicable for detecting and moni-toring condensation within a wall or roof assembly and in testapparatus.1.3 Exposure site calibration or characterization can besignificantly enhanced if TOW is measured for comparisonwith other sites,particularly if this data is used in conjunctionwith other site-specific instrumentation techniques.1.4 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.5 This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2.Summary of Practice2.1 This practice describes a technique for detecting andrecording surface moisture conditions.The moisture serves asan electrolyte to generate a potential in a moisture sensingelement galvanic cell that consists of alternate electrodes ofcopper and gold,silver and platinum,or zinc and gold.Thespacing of the electrodes may be 100 to 200 m,the widthdimension is not considered critical(Fig.1).However,whenzinc is used as an electrode material,the effects of thehygroscopic nature of the corrosion products on the perfor-mance of the sensor should be kept in mind.Also,the use ofcopper as a sensor material should be avoided in sulfurdioxide-laden atmospheres to avoid premature deterioration ofthe sensors copper substrate.The output(potential)from thiscell is fed through a signal conditioning circuit to an indicatingor recording device.The objective is to record the time thatmoisture is present on the sensing element during any givenperiod.The fact that a potential is generated is critical to thistechnique.As pertains to this practice,the absolute value of thepotential generated is essentially of academic interest.2.2 This practice describes the moisture-sensing element,procedures for conditioning the elements to develop stablefilms on the electrodes and verifying the sensing-elementfunction,and use of the element to record TOW.3.Significance and Use3.1 This practice provides a methodology for measuring theduration of wetness on a sensing element mounted on a surfacein a location of interest.Experience has shown that the sensingelement reacts to factors that cause wetness in the same manneras the surface on which it is mounted.3.2 Surface moisture plays a critical role in the corrosion ofmetals and the deterioration of nonmetallics.The deposition ofmoisture on a surface can be caused by atmospheric or climaticphenomena such as direct precipitation of rain or snow,condensation,the deliquescence(or at least the hygroscopicnature)of corrosion products or salt deposits on the surface,and others.A measure of atmospheric or climatic factorsresponsible for moisture deposition does not necessarily givean accurate indication of the TOW.For example,the surfacetemperature of an object may be above or below both theambient and the dew point temperatures.As a result conden-sation will occur without an ambient meteorological indicationthat a surface has been subjected to a condensation cycle.3.3 Structural design factors and orientation can be respon-sible for temperature differences and the consequent effect onTOW as discussed in 4.2.As a result,some surfaces may beshielded from rain or snow fall;drainage may be facilitated orprevented from given areas,and so forth.Therefore variouscomponents of a structure can be expected to perform differ-ently depending on mass,orientation,air flow patterns,and soforth.A knowledge of TOW at different points on largestructures can be useful in the interpretation of corrosion orother testing results.3.4 In order to improve comparison of data obtained fromtest locations separated on a macrogeographical basis,a1This practice is under the jurisdiction of ASTM Committee G01 on Corrosionof Metals and is the direct responsibility of Subcommittee G01.04 on AtmosphericCorrosion.Current edition approved Jan.1,2012.Published March 2012.Originallyapproved in 1981.Last previous edition approved in 2005 as G8489(2005).DOI:10.1520/G0084-89R05.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 uniform orientation