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Blasting and Coating Cycle

June 14, 2011

 

Blasting and coating/painting is done to protect structures  from rust and corrosion. All types of material viz. steel, cast iron, aluminum, stainless alloys, concrete etc. needs to be protected from corrosion. Corrosion is the deterioration of a material, because of its reaction with the environment. This disintegration process can be due to chemical oxidation, atmospheric affects, galvanic couple reaction etc. Atmospheric corrosion accounts for more failures on both a tonnage and cost basis than any other type of environmental corrosion. High humidity is one of the main reason leading to corrosion of metal structures.

High humidity can allow blooming or flash rust between the time the surface is blasted visually clean and when the coating is applied, which may result in premature coating failure. This problem can occur on any surface but is most common when there is a large continuous surface as in the interiors of storage tanks, process piping and reaction vessels. In these circumstances, the practical logistics of coating operation prevent the immediate coating of the surface after cleaning.

While the surface is waiting for the coating, small reductions in surface temperature or small increase in the humidity of the surrounding air can result in condensation, which causes immediate corrosion and negates the entire exercise. “Hold the Blast” is commonly used to prevent rust formation between the blasting and coating cycles.

General Recommendation:

  • Maintain a 5°F (3°C) differential between the air dew point and surface temperature of the steel surface to be coated, with the surface temperature being high. 
  • Maintain a 10°C Dew point differential between the air inside and the air out side of the tank. 
  • Maintain relative humidity of the surrounding air below 40% 

All Major specifications call for above conditions including :

  • EIL       : “Standard specs for shop & field painting”
  • NACE  : “6A 192”
  • SSPC  : “SP 10”
  • BS       : “BS 4232”
  • SS       : “SIS-055900-1967/ISO-8591-1-1988

TDS Solution:

Dehumidification is the simplest and most cost effective solution to ensure proper surface preparation and coating. Dehumidification removes the moisture from the coating system. Dehumidifying the air inside the tank while blasting and coating helps to achieve and maintain the level of moisture to prevent condensation and thus, rust bloom.

An adequately designed Dehumidification system during blasting and coating operations ensure that :

  • The air relative humidity inside the tank is maintained at the level as specified by the coating manufacturer, for optimum performance of the coating enabling timely completion. 
  • The air dew point is maintained at least 5°F (3°C) below the surface temperature to prevent condensation and flash – rust, the common reason for premature coating failures. All specifications including NACE, SSPC,EIL etc. call for the same.. 
  • Prevents any condensation between coats there by reducing the possibility of inter coat de lamination and improving curing properties. 
  • Prevent builds up of hazardous and inflammable vapors inside enclosed areas being blasted and provide ventilation air for site personnel. 
  • Assured quality coating, adhering to paint manufacturers specification 
  • The life of the coat is increased by 1.5 times-2 times 
  • Inter-coat adhesion is appreciably improved 
  • Coating is possible at any time of the year 
  • Down time can be accurately programmed 
  • Project time and cost is reduced by 35% 

 

TDS provides desiccant based dehumidifiers,which offers ideal solution to moisture & humidity control and helps in maintaining the above specified conditions. This helps in preventing rust and corrosion formation and increases life of coating. Along with dehumidifiers TDS provides condensing unit,blower,ducts and skilled manpower for services on RENT.

 
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