Invisible water vapour in the air. Father of rusty bolts and industrial foe of product quality & storage.
A certain amount of water vapour is always present in the air. We experience a relative humidity of 50% at comfort temperature 77-80°F as pleasant. In India, in most parts of country the RH varies between 60-90%, corresponding dry belt temperature range from 85°F-100°F. The effects of this humidity are felt by man as well as machine. We accept and forget about the ever present humidity as part of our life, but the effects of humidity on the machanized industry are long term and devastating and cost a company thousands of rupees every year for maintenance.
The destructive effects of moisture can be simply explained as when the air reaches its dew point temperature then at temperatures just below dew point, condensation takes place resulting in depositing droplets of water on the cold surface. This condensed moisture reacts with certain inorganic materials or acts as a medium conducive for substances to attack materials.
The Critical Humidity
The rate of corrosion attack or rusting is considerable above a certain relative humidity. But below the critical humidity the rate of corrosion is negligible. The critical humidity in most cases is about 45%. In some cases even lower, however in practice the relative humidity to be maintained for storage of steel components and machinery should not exceed 40%.
The graph shows the rate of corrosion accelerating above 45% RH.
Hence it is imperative that the RH must be controlled. It is possible to achieve a low RH by either heating or dehumidifying the air. When the air is heated, the temperature of the air is raised. The equipment to be protected must also be heated to the same temperature as air, because the dew point temperature is very high & if the metal surface is lower than surrounding air, condensation will begin. Heavier equipment which takes a long time to heat up is not given proper protection during the time it is being heated.
Moreover the moisture content & dew point temp. during heating remains unchanged, whereas dehumidification reduce the moisture content and dew point of air at the lower ambient temperature.
Hence the solution to your corrosion problems is to tackle Moisture & not temperature.
Hence dehumidification becomes extremely important in eliminating corrosion in-
- Storing large equipment or small machinery parts over any length of time.
- During lay up or standby of large machinery or ships in docks.
- For cleaning or coating application before painting.
For machinery including vehicles & large mechanical systems susceptible to corrosion, storing them in a room where relative humidity is controlled within a range of 35% to 45% will keep them from getting rusted & corroded.
Associated Bearing in their storage room for Ball & Roller bearing maintain a RH of 45% & temperature 37°5C±1°C with a 600 cfm Bry-Air dehumidifier.
In installations, ships or near the seashore facilities where humidities are as high as 80 to 90% throughout the year dehumidification is essential for maintenance. Sustained exposure to humid conditions and atmospheric suspended salt will deteriorate, in time, the costly & sensitive equipment.
Blasting & coating operation on ship's tanks, large plants etc. often are impossible without control of humidity levels. Metal surfaces begin to rust immediately after being sand blasted in presence of high RH or when the surface temperatures are low. The coating procedure has to be repeated & the paint does not adhere properly.
Having pointed out the distinct advantages of dehumidifier over storing by heating air which is tar more costlier, dehumidification does give you the proper solution to moisture problem & hence corrosion problem.