46 MATERIALS PERFORMANCE December 2012 NACE International, Vol. 51, No. 12

C L B L O G

Continued from page 45

ture, and 3) acidic vapor sources. A

proÅled surface will hold more soda than

a smooth surface. High humidity and

rainfall will supply more moisture than a

desert environment. Carbon dioxide

(CO2) forms carbonic acid (H

2CO

3) on

contact with moisture, but acidic fumes

from chemical processes may be more

problematic.

If the vessel surface has a sharp angu-

lar proÅle acceptable for the coating to be

applied, no additional work is required.

The problem is to verify the proÅle depth

and shape. Depth is easy. ProÅle shape

is more difÅcult. An additional concern

would be verification that the soda-

blasted surface is free of soluble chloride,

nitrate, and sulfate salts.

Coating flange face

QI need to apply an in-

terior epoxy lining to

steel pipe for a water

authority project. The

pipe is carbon steel with a bolted Æange

connection. My instructions are to coat

only to the welded corner of the pipe wall

to the Æange intersection. When bolted

into place, the gasket between the pipe

Æange and a mating Æange leaves a small

area of uncoated steel at each joint. I

believe the coating should be extended

onto the Æange face to provide protection

there. Engineers counter that any coating

on the Æange face will affect the gasket

seal. Which practice is more acceptable?

AJust tell the engineer to have

plenty of spare Æanges on hand

when he has to go back and re-

pair the corroded Æanges when

they start to leak! I hope it’s not my drink-

ing water.

AIf the water is potable, I hope the

coating is certified for use for

potable water. Also, the small

uncoated area will corrode

quickly because of the small anodic area

present and the high current density that

will develop.

ACoating Æange faces has always

been a hot discussion between the

end users and contractors. Some

end users are not willing to go for

such an option while others push for it.

Some important factors, which must be

considered while dealing with flange

faces, are:

• If you are dealing with a Æat face,

normally pressures are low and

rubber or non-asbestos compressed

gaskets are used. Even if you have

an uneven surface, the gasket can

provide the cushion to hold the

pressure. It is common to extend

the coating on the face.

• If you are dealing with a raised face

or ring-type joint Æanges, normally

pressures are quite high. In this

NACE International, Vol. 51, No. 12 December 2012 MATERIALS PERFORMANCE 47

case, termination of the coating can

be carried out in two different ways.

One is by a rebate groove, which

may extend to a length of 3 to 5 mm

on the Æange face and with a depth

of 500 to 1,500 µm, depending upon

the coating material used. The sur-

face can be ground manually back

to the original dimensions once the

application is dry. It covers the initial

crevice where you may expect corro-

sion. The second option is normally

suitable for new installations, where

the Æange is applied with the coat-

ing and machined to the dimensions

prior to the fabrication required. I

have used similar applications on

oil and gas separators at >1,000 psi

(6.8 MPa) with no leaks afterward.

• The most important point that must

be considered is the type of coating

system; if it does not have sufÅcient

compressive strength and Æexibility,

it will crack during the bolt ten-

sioning. Fibers or thin mat can be

combined to enhance the properties

of a coating system. Rubbers, once

applied on water boxes and similar

areas, are good for low pressures

and temperatures. The same mate-

rial applied on a high-pressure and

high-temperature area may deform

over a period of time, creating a bare

spot that may lead to a leak.

AMy preference would be to carry

the coating over the Æange. The

gasket should have enough com-

pressibility to give an effective seal.

When we were making polysulÅde rubber

in one of our reactors, we had to do exactly

that. When we Årst lined the reactor, we

stopped the coating at the Æange edge. We

had signiÅcant corrosion, even though we

were using an exotic alloy tank. We re-

lined the reactor and lid. We brought the

coating over the Æange face for both the

lid and the tank. When we sealed it with a

gasket 2.25-in (57-mm) wide by 0.25-in

(6.4-mm) thick, we had no further corro-

sion. When we shut down the operation

after 23 cycles, there was no corrosion

anywhere.