Difference Between Dew Point 5°F ]QOf 15°F and Surface Temperature (-15° C) (-12.2o C) (-9.40 C)

Approximate RH at the Surface 82-86% 74-80% 54-62%

Table 1. Dew Po int Different ials and Surf ace RH

coatings manufacturer's data sheets

state that a five- to seven-degree dew

point spread is required to provide

adequate protection against condensa­

tion. However, NACE International and

Society for Protective Coatings (SSPC)

have a joint guideline (6A192/ TR-3:

Dehumidification and Temperature Control

During Surface Preparation, Application,

and Curing for Coatings/Linings of Steel

Tanks, Vessels, and Other Enclosed Spaces)

that clearly identifies a much safer range

of dew point spread and maximum

surface relative humidity (RH). It states,

"The typical minimum dew point differ­

ential specified for holding a blast over

an extended period of time is 17-25• F with a rela tive humidity n o t to exceed

40-55 percent. It has been found t hat

rust bloom accelerates if the surface RH

exceeds 40-55 percent RH." So which

is it?

The Two Views Often the 5- 7· F differential range is

intended to provide a sufficient dew point difference to avoid condensation,

enough time to clean up and coat without experiencing condensation on the fresh

coating, and to avoid flash rusting on

freshly blasted surfaces. 1he recommen­

dation also includes the direction that

the surface should not exceed 80 percent

RH. However, this range does not allow

for cool-weath er changes, s udden down

pour of rain, or long periods of holding the blast in order to accommodate the

application of monolithic coatings.

Specifying only a dew point spread is

insufficient to ensure that you will meet

the surface relative humidity specifi­cation. The risks are too great and an

ovemight rust-bloom is too costly.

The NACE 6A192/SSPC-TR-3 speci­

fication for the use of dehumidification

and other climate control equipment, Write in Reader Inqu iry #119

20°F 25°F 3Q0f 35° F (-6.7° C) (-3.90 C) (-1.10 C) (1.7° C)

43-51% 34-42% 27-35% 19-26%

COATINGSPRO MARCH 2016