Problem Solving Forum
Effect of RH on the Time Between Blasting and Priming

From JPCL, October 2015

How long can a blasted surface be left before priming under different temperatures/relative humidity environments?...
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Tagged categories: Humidity and moisture; Paint application; Primers; Quality Control; Surface preparation

Comment from DHIRENDRA KUMAR, (11/21/2015, 5:04 AM)

Irrespective of environmental conditions, blast cleaned surface must be coated with primer before oxidation commences. In no case cleaned surface should be left overnight.

Comment from Pieter van Riet, (1/6/2016, 5:22 AM)

Many legacy specifications still prescribe a maximum allowable delay between final blast cleaning and coating application, often as short as 4 hours. This is technical non-nonsense, gives a false sense of comfort, and only serves to drive up the cost of corrosion protection. It forces the contractor to blast, coat and dry multiple small blocks, with risk of disruption and delay between each block, and increases the risk of contaminating the drying coating with blasting dust. Water-soluble micro contaminants remaining on the surface after blast cleaning is the root cause of flash rusting, not the water vapour in the air. The water vapour only serves to dissolve the soluble contaminants on the substrate, which greatly increases the conductivity of the water vapour and thus the effectiveness of the electrolyte, completing the corrosion cell. The more contaminated the substrate, the quicker it flash rust. The second issue to consider is is that polymer-based coating materials are substantially permeable to water vapour. Water- and other small molecules readily diffuse into a dry coating film, and out again, reaching an equilibrium. However water-soluble contaminants such as environmental or metallic salts that remain on the substrate under the coating film will upset this equilibrium. It will dissolve into the diffusing water vapour, trapping the water molecules under the coating. It will slowly form a highly concentrated and very effective electrolyte UNDER the coating, leading to the typical measles rust spots and under-film corrosion. So what is the point of trying to coat the steel quickly before it flash rusts, when this flash rust is itself an indication of improper surface cleaning and harbinger of future problems? You are only delaying the pain. Instead of focusing on avoiding flash rust, the specifier's aim should be to ensure the substrate is PROPERLY cleaned before the coating is applied, in order to achieve long term maintenance-free corrosion protection. This requires special attention and effort to remove environmental- and metallic salt decontamination, and proper testing to confirm it. The use of a proprietary (low pH) decontamination product has proved to be very effective in rapidly reducing contamination to very low levels. If it is not possible or economical to reduce water soluble contamination to an acceptable level, increasing the barrier properties of the coating system should be aimed for. This can be achieved by specifying a coating with superior vapour permeation resistance (such as a properly formulated glass-flake coating), and/or increasing the specified dry film thickness. By applying these principles we have multiple case histories for projects that achieved coating service life approaching 30 years, including in seawater immersion service such as pump internals and riser pipe lining.

Comment from Marco Antonio Alvarado Meneses, (1/15/2016, 12:11 PM)

It dependens on several factors: %HR, Dew point and surface contamination. It is very difficult to establish 04 hours between SSPC SP 10 and priming, then, this time must be established in site.

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