1. Introduction
Development of coating technology was given a boost during World War II. When it became clear that improved corrosion resistance could be achieved with metal structures fabricated and coated indoors. More advanced coatings were developed with even better corrosion resistance if applied indoors. When applied outdoors, the corrosion resistance of the coating was dependent, to a great degree, upon weather conditions during applications.
Surface condition is now more important than ever when improved paint specifications increase the degree of corrosion prevention expected by the end-user. Unfortunately, a high performance coating develops its full protective potential only when applied to a very high quality surface. Getting metal surfaces to this high degree of physical and chemical cleanliness and then keeping them clean until painted is now a technology in itself.
The purpose of cleaning a metal surface-the metal usually being steel by dry abrasive blasting is to remove old paint, rust, and scale without removing metal.
Providing any oil-based chemicals and general dirt have previously been removed, a blasted surface should then be clean to currently accepted standards. Paint application must then be completed before the surface loses its characteristic gleam, and tell-tale blooming indicates that “the blast has gone off.”
This loss of the bright surface appearance is due to the formation of a fine layer of oxide by atmospheric corrosion. Paint can be applied to such a surface, but the corrosion, once started, may continue unseen under the coating, significantly reducing its useful life. For this reason, specifying authorities are placing more and more importance on the inspection of the metal surface prior to coating, in which the full potential of today’s coatings may be achieved.
2. The Corrosion Mechanism
Corrosion theory is a complex subject, so fortunately only a small part is relevant here. What is of interest is the mechanism of atmospheric corrosion and hence the factors controlling the initial rate of deterioration of the bright surface. By reducing the initial corrosion rate, the time allowable between cleaning and painting can be increased without excessive deterioration of the blasted surface.
For corrosion (an electrolytic process) to occur, there are three necessary conditions:
- Heterogeneous material, i.e., variations in material composition which can act as respective anodes and cathodes;
- Electrical contact between anode and cathode, allowing current to flow into the anode;
- The presence of an electrolyte (usually water) making contact with anode and cathode at the same time.
All three conditions need to be satisfied for corrosion to take place. To inhibit corrosion, only one of the three factors has to be absent.
Since anode and cathode are both part of the steel, they cannot be electrically isolated.
Thus, with plenty of anodic and cathodic sites and good electrical contact between them, the component which in practice limits the corrosion rate is the electrolyte.
Research work, principally be Vernon in the 1930’s, showed that three factors above all determine the rate of atmospheric corrosion of steel: steel temperature, air humidity, and atmospheric pollution. Steel temperature changes the corrosion rate in the same way as temperature affects the rate of most chemical reactions. Air humidity and atmospheric pollution, on the other hand, control the corrosion rate by affecting the ability of the electrolyte to transfer ions (conduct corrosion current). This is illustrated by explaining the following observed corrosion phenomenon in terms of electrolyte function. Steel was observed to corrode 15 to 25 times slower in a hot, wet Nigerian jungle than in various industrial cities in England. One difference is the degree of air pollution experienced at the two sites.
In the jungle, the steel surface may have been covered in a film of water, but it would have been relatively clean water with few dissolved ions, due to the lack of atmospheric pollution. Such water does not form negative ions easily so the slow cathode reactions would have limited the corrosion rate.
By contrast, atmospheric pollution in industrial areas causes any water present on the steel surface to become conductive, which in turn greatly increases the rate of formation of negative ions at the cathode. Direct attack by the pollutant, notably chlorides and sulfur dioxide, on the steel is also known to occur.
Either way, moisture is a prime factor in the corrosion.
Talk of water on the steel surface may give a false impression. Presence of water does not necessarily mean that the surface feels wet. Contaminants can absorb water from the air and establish a corrosion cell. It is therefore a mistake to think that keeping a surface apparently dry by stopping condensation is sufficient to stop corrosion. To stop corrosion it is necessary to keep the air dry enough to prevent the contaminants on the steel surface from absorbing water. In order to go further and present the humidity criteria for retardation of corrosion, some knowledge of humidity is desirable.
3. A Little Bit About Humidity
Air is a mixture of gases principally nitrogen and oxygen, but it also contains a number of other gases in small quantities, of which water vapor is one. The amount of water vapor that the air can hold is limited and depends on temperature; warm air can hold more vapor than cold air. When the air contains the maximum amount of water it can at any given temperature, it is said to be saturated. If it contains half that amount, it is said to be 50 percent saturated, or to have a “relative humidity” of 50 percent. Air can therefore have a relative humidity of between 0 percent (perfectly dry) and 100 percent (saturated).
Increasing the temperature of the air by heating does not increase its “moisture content”-its weight of vapor-but does increase the weight of vapor it could hold at saturation. Heating therefore decreases relative humidity although no vapor has been removed.
By cooling the air, the saturation level is reduced and the relative humidity increased. If cooled far enough, it must eventually become saturated . If cooled further, the excess vapor that the air cannot now hold must condense, either as mist of fog or as droplets of dew on any convenient surface. Whatever the humidity, it is always possible to cool the air sufficiently to achieve saturation and then condensation. This temperature at which the air has been cooled far enough to be saturated and on the point of producing dew is called the dewpoint temperature.
Another definition of dew point is that temperature at which saturated air contains just the same weight of water vapor as does the air in question.
The relationship between temperature, relative humidity, and dew point can be found in tables or by using a psychrometric chart (Fig.1). One important use of the psychrometric chart is illustrated be the following example.
Air at 17 C, 50 percent relative humidity, is cooled to 11 C. To what level does the relative humidity increase? Fig. 1, point 1, shows 17C and 50 percent relative humidity. Dew point can be read off as 6.6 C, which means that this air contains just the same weight of water vapor as does saturated air at 6.6 C. This new air condition is shown as point 2. The new relative humidity can be read off as 74 percent.
It should be noted that if the air had been cooled to below its original dew point of 6.6 C, say to 3 C, then the new air condition would have been saturated at 3 C, that is 100 percent relative humidity with a dew point also of 3C. This condition is shown as point 3.
The weight of vapor would also have been reduced and dew would have been formed on a convenient surface.
4. Effects of Humidity on Corrosion Rate
In the previously mentioned work, Vernon found that, apart from high atmospheric pollution, high air humidity promoted fast corrosion. He found that air relative humidity was the critical factor, with corrosion rate being very low below about 60 percent relative humidity but much faster above (Fig.2). The sharp “cut off” or “knee” in the graph at about 60 percent is the vital factor. Normal atmospheric air relative humidity is well above 60 percent, typically 75 percent to 80 percent. Comparing the corrosion rate at, say, 50 percent to 55 percent with that at 75 percent to 80 percent shows that if the relative humidity is kept at this lower level the steel corrodes several times more slowly than would normally be the case.
A relative humidity of 60 percent is therefore the upper limit for minimal corrosion. A practical figure has proven to be 50 percent, being not only easy to remember but also affording a small safety margin. By using relative humidities as low as 30 percent, though, blast-cleaned surfaces have been held even longer before coating.
It is too easily forgotten that the corrosion rate is determined by the relative humidity of the air in contact with the steel surface. This is frequently quite different from the relative humidity of the air even a few millimeters away from the surface and happens if the air and the steel are at different temperatures. Unless water is evaporating from or condensing on the steel,the air close to the steel-the air “boundary layer” will be in moisture equilibrium with the surface. With no net transfer of moisture content of the air next to it, that air will in turn assume the moisture content of the air next to it, and so on, with the result that the boundary layer air moisture content will be the same as that of the main body of air well away from the surface.
Temperature equilibrium is less likely. The boundary layer air assumes the temperature of the steel surface, and heat then flows from this boundary layer into the main body of air, or vice versa.
To repeat, corrosion rate is controlled by the relative humidity of the air in the boundary layer. This cannot be measured directly because practical sensors cannot get close enough to the steel surface. The boundary layer is thus assumed to have the same moisture content-and hence the same dew point as the main body of the air, but the same temperature as the steel. The psychrometric chart is extremely useful here, for any method can be used to measure the condition of the main body of air away from the surface, and hence its moisture content. A contact-type surface thermometer suffices for measurement of steel temperature. The following figure gives a practical example of such a calculation.
In this example, the dry and wet bulb temperatures of the main body of air are measured ( by a sling psychrometer) to be 8.6 and 4.9 C, respectively. This equated to a relative humidity of about 70 percent for the surrounding air. The relative humidity in the boundary layer adjacent to steel depends on the steel surface temperature. This is measured as 2.1 C. From the chart this condition corresponds to a relative humidity of about 87 percent.
The fact that the corrosion rate is determined by a combination of air moisture content and surface temperature leads directly to the conclusion that there are only two methods of reducing the boundary layer air relative humidity. One is to reduce the moisture content by dehumidification and the other is to raise the surface temperature.
5. Relative Economics
It is theoretically possible to control relative humidity at the steel surface through either moisture content or surface temperature. Which method is chosen depends on the relative cost. With small areas it is often possible to raise the surface temperature using a radiant heater, while for large structures, particularly enclosed surfaces such as the inside of oil storage tanks, large numbers of radiant heaters would be required to cope with the heat losses from the steel surface to the outside air.
One technique that is sometimes used but that is decidedly uneconomical is to heat the air itself. This does not alter the air moisture content, so to lower the boundary layer air relative humidity, the heating must raise the surface temperature. In the worst case, this is virtually impossible because of poor heat transfer from air to steel and also because of the steel’s large heat capacity. At best, most of the heat put into the air goes to waste and only a few percent into the steel.
6. Selecting Dehumidification
The dehumidification solution to the problem of corrosion is to reduce the moisture content of the air to a safe level, typically 50 percent relative humidity or less at the steel temperature. A flow of air through the working area is required, and this air has to be dehumidified.
If the source of the air is the atmosphere in other words fresh air, which is usual its moisture content needs to be changed from that of the fresh air to the equivalent of 50 percent relative humidity or less at the steel temperature.
In selecting the right type of dehumidification for the job, emphasis should be put on the low temperature performance of the machine. Refrigeration dehumidifiers are commonly used in industry and have the capability to remove large amounts of water at high temperatures and humidities, but do not perform well under low temperature winter conditions when the water has to be frozen out of the air, rather than condensed as a liquid. Absorption, or “chemical” dehumidifiers, on the other hand, have nearly the same performance summer and winter, as the affinity of the chemical absorbent for moisture is not greatly affected by temperature.
Fig. 3 shows a comparison between “equivalent” refrigeration and chemical dehumidifiers at different times of the year, emphasizing the superiority of the chemical dehumidifier in winter.
7. Benefits of Dehumidification to Contractors
Instead of the daily blast-then-coat cycle (blast and coat only what can be done in one day) contractors have the option of using dehumidifiers to dry the air in a tank before blasting and coating. Now a contractor can dry the air in a tank, blast the entire surface, hold the blast with dry air, clean the surface, and then apply coating. Instead of a daily blast-coat, blast-coat cycle, contractors can save time and money with a blast- once, coat-once routine. Besides the obvious benefit in time savings, there are added benefits to this process.
Crews can start working early in the morning they do not have to wait for the right weather or temperature conditions. Usually, this means waiting for morning fog conditions to clear or for the air temperature to rise and warm the surface of a tank. Overlaps from one coated surface to the next in the daily blast-coat outine are eliminated. Coatings are applied once when the surface is ready. Dust and grit that typically fall on a newly coated surface in the daily blast-then-coat routine are eliminated. Coatings are applied once when the surface is ready. If more than one coat is being applied, then they all can be done under ideal conditions. The typical “two day window” between primer and final coat can now be achieved under optimum conditions.
*Note: Once a contractor applies the primer or first coat, he has a limited amount of time to get the second coat on. The window starts to close in a sense. If the primer goes beyond the window and becomes too hard, then the second coat will not adhere as well.
Contractors can guarantee when a tank will be completed. Without dehumidification, contractors are at the mercy of changing weather conditions and can work only when the weather is right. Dehumidification can extend the season by two months in some areas. Contractors typically will not start work in the spring until the weather is dry. Dehumidification will allow them to work earlier in the spring and later in the fall when weather conditions change again. Dehumidification can help eliminate the backlog some contractors experience when they cannot finish all of their work in the fall. This is especially true in the North. Contractors will then carry the backlog through the winter and start those jobs as soon as they can in the spring.
The net result of these benefits is that the contractor is now in control and is not the victim of weather and atmospheric changes.
