When we in surface treatment terms talk about “ambient conditions”, we actually refer to the existing climatic conditions at the scene of the job. The reason for this is that these conditions will have a decisive influence on the resulting quality of the surface treatment, both for the surface preparation, the installation (application) and the conditioning (curing) of the treatment.

The climatic (ambient) conditions we are interested in are not only whether the sun was shining, if it was cloudy or if it was raining, but more specifically:

• The temperature of the air at the scene
• The temperature of the surface receiving the treatment
• The relative humidity in the vicinity of the work
• The risk for condensation to take place on the work surface

The reasons for this interest are that many products and procedures used for surface treatment have restrictions regarding their use at different temperatures and may be sensitive to high or low humidity at critical stages, as well as unsuitable if the surface to be treated is moist or wet.

Actual humidity is the actual water content in the air, expressed as gram of water per cubic-meter air (g/m³).

Maximum humidity is the maximum amount of water vapour which can be contained in air. This will be different for different temperatures, since warm air can hold more water vapour than colder air:

• At 10^oC, the maximum moisture content in the air is 9.4 g/m³
• At 20^oC, the maximum moisture content in the air is 17.3 g/m³

Relative humidity is an expression for how many percent the actual humidity in the air is of the maximum humidity at the given temperature.

• If the relative humidity is 100%, this means that the air contains the maximum water vapours it possibly can have at that temperature (i.e. actual and maximum humidity are the same).
• If the actual humidity is 30 g/m³ and the maximum humidity at that temperature is 60 g/m³, the relative humidity will be 50% (30 is 50% of 60)
• If the temperature of above air with actual humidity of 30 g/m³ should increase, the actual humidity will remain the same (30 g/m³), but the increased temperature means that the maximum humidity will also increase. If the maximum humidity increases from 60 g/m³ to 75 g/m³, the relative humidity will drop from 50% to 40% (30 is 40% of 75).
• If the temperature of the air in the example with 30 g/m³ should decrease, the actual humidity will remain the same (30 g/m3), but the decreased temperature means that the maximum humidity will also decrease. If the maximum humidity decreases from 60 g/m³ to 50 g/m³, the relative humidity will increase from 50% to 60% (30 is 60% of 50).
• If the temperature of above mentioned air drops so low that the maximum humidity at that temperature is only 20 g/m³, we would in theory end up with a relative humidity of 150% (30 as percentage of 20). It is however not possible for the relative humidity to be higher than the maximum humidity. As a consequence, the overload of water vapour (humidity) will be expelled from the air in form of condensation.

Temperature in the air: Unless special heating or cooling facilities are used, paint still in its container (tin) will adjust to the air temperature where it is stored. Some paints are required to be at a minimum temperature when being used, such as solvent free paints (for spray application).

Substrate temperature: Regardless of the temperature paint in its tin, as soon as it has been applied to a substrate it will very quickly adjust to the temperature of the substrate. Some paints require a minimum temperature in order to cure, such as epoxy paints.

Drying times (such as dry-to-touch, dry-to-recoat, dry-to-use, cured, etc.)will always be influenced by the prevailing temperature, and must for this reason always be checked and recorded.

As explained above, if air contains its maximum humidity content, the RH will be 100% and it is on the borderline for condensation to start. Dew point is what we call the temperature at which the RH is 100% and any excess moisture in the air is expelled in the form of condensation (or dew, as sometimes seen on window panes and mirrors).

Example:

• On an early morning inspection, we measure the air temperature to be 8^oC and the RH to be 83%
• Due to cold over-night temperatures, the steel temperature of the structure to be painted is measured at 4^oC

This would cause a problem

• At 8^oC and RH 83%, the moisture content is 6.9 g/m³
• Although the general air temperature is 8^oC, the air next to the steel surface will be cooled by the steel and perhaps only be 4^oC as well
• At 4^oC the max moisture content is 6.4 g/m³
• This means that although we may not see it clearly, there will be condensation on the steel (6.9 g/m³ actual versus 6.4 g/m³ maximum: excess moisture content!)

The danger for condensation is not only present on early mornings. A sudden change in weather can increase the possibility for condensation to take place at any time of day or night. To ensure that there is no condensation on the substrate to be treated, the sensible work practise is to identify what the dew point is under the current conditions and check that the substrate temperature is higher than the dew point. If the substrate temperature is below the dew point, it is almost certain that the substrate is moist or wet from condensation, even if it may not always be easy to see this with the naked eye.

See below for how to carry out these measurements.

Why are we so concerned about condensation?

• Condensation is water from humidity in the air
• Water on a surface prior to application of paint may lead to adhesion failure
• Water on an abrasive blasted steel surface will cause corrosion
• Condensation on the surface of freshly applied paint may lead to a paint failure or loss of aesthetics (blushing or blooming)
• ISO 8502 Part 4: Guidance on the estimation of the probability of condensation prior to paint application states that “Unless otherwise agreed, the steel surface temperature generally should be at least 3^oC above the dew-point when paints are used.”
• Above RH 85% the risk of developing condensation on the surface to be painted or condensation on the not yet dried paint film is high, so the industry does not recommend application of paints above RH 85%

While assessing the probability of condensation on the outside of larger object, it is important to bear in mind what is hidden behind the surface:

• Cold liquid in a tank behind the surface and warm air outside will cause condensation on the outside surface
• The mass of steel, such as frames and stiffeners inside the structure, can keep the steel cold longer in very localised areas

Corrosion is clearly influenced by ambient conditions:

• Atmospheric corrosion increases at higher humidity, above RH 60% there is a clear acceleration, but there is very little corrosion taking place in very dry areas (desserts)
• Since corrosion is a chemical reaction, it speeds up at higher temperatures and slows down at lower temperatures (high in the tropics, low at Antarctica)
• Consequently, corrosion should be very high in tropical rainforests

Paint temperature

• Low temperatures will increase the viscosity, making application difficult (poor flow, poor atomization)
• Higher temperatures can lead to a dry-spray finish and shorter pot-life

Substrate temperature

• Needs to be at a temperature that allows the paint to dry / cure
• Needs to be at a temperature that allows the paint to flow and form a continuous film
• For drying/curing the substrate temperature is more important than the paint temperature
• The paint will acquire the temperature of the substrate after application

Relative humidity

• High humidity may interfere with some paints drying and curing, causing blushing and blooming
• Low humidity may prevent some paints from curing (e.g. inorganic ethyl zinc-silicate and cement based paints)

Ambient Temperature and Steel Temperature

Air temperature will influence:

• Shelf life (storage)
• Pot life
• Induction time
• Viscosity/sprayability
• Steel Temperature

Steel temperature will affect:

• Probability for condensation
• Solvent evaporation
• Viscosity
• Recoating interval
• Speed of drying/curing
• Service life of the coating

It is not feasible to control ambient conditions in general. It is however possible to influence them in enclosed or confined spaces, such as inside tanks. These are some of the options:

Dehumidifier

Reduces the actual humidity in the area / surroundings (must be enclosed space)

Heater

Increase the temperature of the objects (surfaces) prior to application in order to reduce the danger for condensation

Forced ventilation

Good ventilation is necessary in order to secure sufficient evaporation of the solvents from the paint film. Temperature and humidity of air used for drying must be monitored:

• Supply of heated air immediately after application may lead to skin drying and entrapped solvents
• Cold air will keep the film open longer and ensure proper evaporation
• Avoid high air temperature (especially epoxy)
• High humidity will slow down drying of water based paints
• Exhaust from heating equipment using propane or paraffin oil contains water and carbon dioxide and may cause amine sweating.

Solvent vapours are heavier than air, so suction ventilation should be arranged at the lowest points in enclosed areas.