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Hold The Blast

August 1, 2011

 

Corrosion of surfaces can occur when wet surfaces comes in contact with atmospheric oxygen. High atmospheric humidity enhances condensation of moisture on the surface. Condensation result in flash-rust causing the coating to fail. 

Most of the failures of coating are a result of poor or inadequate surface preparation. The objective of surface preparation is to create proper adhesion of coat over the substrate

Any surface after it is blasted and prepared for coating is subjected to environmental factor, until the first coat is applied. Presence of high humidity condition causes the formation of rust bloom on the mental surface, coating over which would lead to blistering and de-lamination. 

The term “Hold the Blast” is commonly used and referred to the prevention of rust bloom from forming between the blasting and coating cycles. It also refers to  preventing flashing or immediate corrosion after blasting the metal surface, commonly referred to as maintaining the ‘white metallic luster’.Proper environmental controls with Dehumidifiers help in ‘holding the blast’ during the blast coat cycles. 

The life of the coating cannot be guaranteed on large, cool metal surfaces like ship interiors, petroleum and chemical storage tanks, etc. unless the surface is clean and dry when the coating is applied. This is because blasting and coating operations on metal surfaces encounter several problems, when RH is high or surface temperatures are low. 

High humidity in the environment, condenses on the colder surface in the form of droplets, causing the metal surfaces to rust immediately after being blasted. As a consequence, the coating does not adhere properly and coating process has to be repeated. Besides this, condensation also causes blistering, blooming, peeling problems in paint jobs. 

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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