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The quality and usefullife of anti-corrosion painting is determined by several actors. The pretreatment

and the protection of the pretreatedsurfaces against moisture are the most important factors.Surf aces which have been blasted or treated in a corresponding manner are completely unprotected againstcorrosion attack and must not be subjected to excessive umidity nor to the risk of condensation.The rate of corrosion attack is considerable above a certain value of the relative humidity - the criticahumidity. But below the critica humidity the rate is negligible. The critical humidity is about 60%,but 50% should not be exceeded n practice.

and +5° C to +15°C is of ten stipulated as he lowest tem-perature for painting. These conditions further limit thepossibilities of carrying out correct anti-corrosion paintingoutdoors.

In order to carry out anti-corrosion painting, it is necessaryto know about the factors which affect and determine theambient humidity, so that this may be checked. When un-saturated air is cooled or its moisture content is increased,ils relative humidity is increased and may continue to in-crease until the air becomes saturated and water vapourbegins to form. The air has then reached its dew point. Inother words, the relative humidity adjacent to a pretreatedsurface is determined by the moisture content of the airand the air temperature at the surface.

Adapting the ambient moistureIt is possible o achieve a low relative humidity by heatingor dehumidifying the air. In adapting the ambient moisturetor anti-corrosion painting, the moisture content of the airor the absolute humidity is of importance, but it is the rela-tive humidity adjacent to steel surfaces which dictateswhether the moisture in the environment is damaging ornot.The important difference between heating air and dehumi-difying it is that while the moisture content and dew pointremain unchanged during heating, the moisture content andthe dew point are reduced by dehum dification (F g. 1).

Indoor paintingIn heated, weil ventilated areas, here is generally sufficient-Iy little ambient moisture for anti-corrosion painting in coldweather. During the cold winter months, the relative humi-dity indoors may even be very low.However, he temperature lag of the material to be paintedmust also be considered. Material which is taken into heated,but undried premises, s usually so cold that moisture con-denses on the surface.During the warm part of the year, the indoor ambient mois-ture is relatively unsuitable for anti-corrosion painting.The indoor ambient moisture may be alternately better orworse than the outdoor environment, depending on the de-sign of the building, its location and other factors. However,the building provides protection against rain and when it hasthermal insulation, it is protected against he daily variationsin temperature, so that the condensation of moisture on ma-terials may be avoided.If an attempt is made to improve the ambient moisture du-ring the summer by heating the indoor air, the heating pro-duces an unacceptably high working temperature. In thesame way, when heating in winter, the risk of a temperaturelag on materials of large mass must be considered.

This difference in principle is important as regards he riskof temperature lag of the materials. When the indoor temp-erature is heated, t may take a long time for the tempera-ture of the materials to reach th at of the air, and when pain-ting outdoor steel structures of large mass, t is practicallyimpossible o raise he temperature of the materials suffici-

Outdoor paintingThe conditions tor carrying out correct outdoor anti-corro-sion painting are very unsuitable. The ave age relative humi-dity is sufficiently low to provide protection against corro-sion only in the afternoons during the summer months.Moreover, the daily variations of the air temperature andthe temperature lag of steel structures must be considered.The critical humidity is easily exceeded and the dew point

of the air is aften reached during the night, resulting in con-densation on steel surfaces. Pretreatment and painting can-not.be started until the surfaces have dried, and must becompleted so that the paint can dry before condensationoccurs again.Of course, anti-corrosion painting cannot be carried outwhile it is raining, nor immediately af er rain.

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doors it is generally easy o achieve a low relative humi-y adjacent to steel surfaces by dehumidification, irrespec-e of the temperature of the materials. Outdoors the desi-d moisture in the environment mayalso be achieved byhumidification, where it is praèticable to erect a coveringound the object to be painted. The covering may be arran-d in one of many different ways. A steel scaffold or woo-n framework cl ad in heavy plastic - possibly reinforcedastic - may of en be sufficient. The cover need not be ab-utely tight, as the demand tor good ventilation must beisfied. During the cold part of the year, dehumidifyingcombined with heating to speed up the drying rate of theaint and o achieve n acceptable orking emperature. Ins context, it should be pointed out that building dryerse not capable f drying, but only of heating he air. soat they only operate as air heaters.

EconomyIn practice it has been found that dehumidifying is conjunc-tion with anti-corrosion painting is clearly profitable. Thecast of dehumidifying is only approx. 4% of the total treat-ment cast of corrosion-preventive painting a ship's tanks.Moreover, dehumidifying is of ten a prerequisite tor the cor-rect execution of anti-corrosion painting. This is the case nthe painting or surface treatment of large structures outdoors,tor instance. The technical consequences f attcmpting toachieve he desired moisture in the environment under thesecircumstances by heating have already been described. The

following example illustrates this and at the same ime showsthe difference in energy casts between heating and dehumi-difying tor the anti-corrosion painting of a storage ank.

Example:The uninsulated storage ank is, in principle, like that in Fig. 3.Volume = 2000 m3Area exposed o ambient a r = 720 m2

Ambient temperature = 12,5° CRelative humidity of ambient air. = 80%Ventilation demand =5000 m3/h

g. 2 shows a mobile dehumidifier tor different types ofutdoor painting. Examples of applications include theinting of ships' tanks and storage anks, bath above andlow ground. Dehumidifiers are also used during paintingrock caverns, pump stations, waterworks and during the

nishing of concrete surfaces n, tor example, water towers.is particularly easy o arrange dehumidification when pain-

ng various types of container indoors, Fig. 3 shows heinciple tor dehumidification while corrosion-preventioninting a storage ank. The moist outdoor air is drawn intoe unit and dried. The air is passed hrough a heater, whereis heated f necessary.

he dried air which may be heated s led into the storagenk through flexible ducts. The air leaves he storage ankrough valves or manholes.s shown in the figure, the air may be filtered and recircu-ted, but is usually blown out of the tank into the air using exhaust fan.

he necessary entilation and the desired values or the rela-ve humidity and the temperature are taken into accounth en selecting he dehumidifier and the heater.he air treatment is carried out during the various phases ofti-corrosion painting; the drying of the tank, blasting,eaning, painting and drying of the paint.

oisture controloth the ambient temperature and the temperature of theeel surface should be measured during the control of thembient moisture.lt may be assumed hat the moisture con-nt is the same near the surface and in the surrounding air,that it is possible o read off the relative humidity of theurface» n the »IX-DIAGRAM».f course t is important to determine the lowest tempera-re of the steel structure to be painted. In doubtful cases,e surface emperature should be checked at severalloca-ons. Variations in the outdoor temperature and humidityust be observed, f the dehumidifying and heating areanually regulated.

The temperature in the tank must be raised o 20° C tolower the relative humidity In the tank to 50%. The tempe-rature of the steel will then be 15.3°C and the relative humi-dity at the steel surface will then be 65%. The temperatureof the steel must be further raised as it is the humidity at thesteel surface which is of importance. It will be found thatthe air temperature must be raised o 32.5° Cso th at tem-perature of the steel will be 20°C and the humidity 50%.To achieve his increase n temperature requires a hot-air

unit with a heating capacity of 120 kW. At an energy castof 0.1 SKr/kWh, this will cast 12 SKr/hour.

The desired humidity in the tank may be achieved using adehumidifier of 25 kW output. At the same energy price,the cast of energy will then be 2.5 SKr/hour.

The calculation does not include the heat lost through thebottom of the tank. The heating demand may be muchhigher in' reality, depending on how th~foundations for thetank are designed. Contact between the bottom of the tankand the foundations or the ground, gives rise to local heatlosses, which mean that the temperature of the steel at thebottom of the tank will be lower and the relative humidityhigher there than in other parts of the tank.

A certain heating effect is also generated during dehumidi-fication, because he drying process eleases nergy in thetreated air. The heating effect may amount to 75% of theenergy consumption of the plant. It is therefore possible nmany cases o achieve he temperature required without theprovision of a separate heater.