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July 12 - July 18, 2015

How long can a blasted surface be left before priming under different temperatures/relative humidity environments?

Selected Answers

From Marco Antonio Alvarado Meneses of Sherwin Williams PerĂº on September 24, 2015:
This is an interesting question, but the answer depends on many factors: % RH per hr, temperature, condition of steel before blasting, pollutants, environment, etc etc. Did you review the paint specification?

If the surface is well protected by means of a dehumidifier, then practically there will be no issues. But still it is a specification deviation. We have to get advanced approval from the client for using the dehumidification unit and extended time interval between surface preparation and primer application.The inspection of surface cleanliness always has to be carried out just before painting, not just after blasting.

From Tolga DIRAZ of CARBOLINE Turkey - Bursa Plant on July 30, 2015:
You may want to look at a study from Munters which I am giving as a reference in my corrosion trainings:

From Anthony Asmar of gulf silicone on July 28, 2015:
 I don't agree that blasted surfaces can withstand thousands of hours at 100% RH without rusting . Otherwise, why would paint manufacturer recommend that surfaces be coated immediately after blasting!!! I will guarantee you that surfaces will start to oxidize and contaminate once blasting is completed. Acceleration of spot/flash rusting will vary depending on RH  and salts on the surface. If you wait overnight after blasting, then inspect the steel surface with a microscope, certainly, the surface cleanliness will be different than it was just after blasting..

From Anthony Asmar of gulf silicone on July 27, 2015:
That's depend on weather condition & percentage of RH, but It is strongly recommended to coat the blasted surface immediately before any oxidation take into place.

From maharajan ramakrishnan of heisco on July 20, 2015:
If the relative humidity is 40%, the blast can be held up to 4 hours. As relative humidity increases from 40%, the amount of time you can hold the blast decreases from 4 hours.

From Tom Swan of M-TEST on July 16, 2015:
A clean surface (free of electrolytes) will not rust. For corrosion to occur,  an electrolyte is required. Pure water (humidity) is a poor conductor and in itself will not cause rust to occur. The problem is, humidity often contains salts from many different sources; and when the humidity hits the steel, eventually, there will be enough salt buildup to cause flash rusting. As far as low RH being sufficient to prevent contaminated steel from causing flash rust, it depends on the species of salts present on the surface. Sodium chloride will pull moisture out of the atmosphere at RH of greater than 75%, magnesium chloride will pull moisture out at 33%, calcium chloride will pull moisture out at 27%. If you have sodium chloride on the surface, 38% relative humidity will prevent flash rust. If you have the other two salts, (along with many others) 38% RH will not be sufficient to prevent flash rusting. Our industry is lucky that the most prevalent salt, sodium chloride, is also not one of the more hygroscopic of the salts.

From alex nunez of tote on July 15, 2015:
There's no time. The answer to your  question is that it depends on temperatures, relative humidity (environmental conditions), the type of substrate you blasted and the service. It will also depend on the results of your inspection by visual observation and use of test instruments and the specification criteria in your contract.

From Warren Brand of Chicago Corrosion Group on July 15, 2015:
If the humidity is low enough, I believe, below 38 RH, steel will not corrode. Jim, I would like to see the report you referenced.

From Jim Johnson of CHLOR*RID International Inc. on July 14, 2015:
Dr Gerald Soltz conducted research  on flash rusting for the National Shipbuilding Research Program in about 1990.  The Executive Summary states that clean steel, even in 100% humidity, will not rust for thousands of hours. That is a true statement that further research has validated.   Only when there was salt contamination on the surface did rusting occur. Also, when salt contamination was present, then temperature and humidity come into play as to how quickly the surface rusts. What this information tells us is that if you desire to have a rust-free surface, you begin by reducing or eliminating contaminants. Many people think that only humidity makes a surface flash rust, but, in reality, contamination is required for flash rusting to occur.

From trevor neale of TF Warren Group on July 14, 2015:
Time is misleading. Steel must be in the condition called for in the specification. The time will vary with local conditions from hours to days.

From Kamalanathan Gnanasekar of Royal Enfield motors ltd on July 10, 2015:
A blasted surface can be left for 12 hours before priming.

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Tagged categories: Abrasive blasting; Asia Pacific; EMEA (Europe, Middle East and Africa); Latin America; North America; Surface Preparation

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