Schweitzer P.A. Fundamentals of corrosion. Mechanisms, causes, and preventative methods

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Corrosion of Polymer (Plastic) Materials 169

between solvent and resin is important. Slightly polar resins, such as polyes-

ters and vinyl esters, are attacked by mildly polar solvents.

Generally, saturated long-chain organic molecules, such as the straight-

chain hydrocarbons, are handled well by the polyesters.

Orthophthalic, isophthalic, bisphenol, and chlorinated or brominated poly-

esters exhibit poor resistance to such solvents as acetone, carbon disulde,

toluene, trichloroethylene, trichloroethane, and methyl ethyl ketone. The

vinyl esters show improved solvent resistance. Heat-cured epoxies exhibit

better solvent resistance. However, the furan resins offer the best all-around

solvent resistance.

Stress corrosion is another factor to consider. The failure rate of glass-rein-

forced composites can be signicient. This is particuarly true of composites

exposed to the combination of acid and stress.

Under stress, an initial ber fracture occurs, which is a tensile type of

failure. If the resin matrix surrounding the failed ber fractures, the acid is

allowed to attack the next available ber, which subsequently fractures. The

process continues until total failure occurs.

5.6 Environmental Cracking

Stress cracks develop when a tough polymer is stressed for an extended

period of time under loads that are small relative to the polymer’s yield

point. Cracking will occur with little elongation of the material. The higher

the molecular weight of the polymer, the less likelihood of environmental

stress cracking, other things being equal. Molecular weight is a function

of the length of individual chains that make up the polymer. Longer chain

polymers tend to crystallize less than polymers of lower molecular weight or

shorter chains, and they also have a greater load-bearing capacity.

Crystallinity is an important factor affecting stress corrosion cracking.

The less crystallization that takes place, the less likelihood of stress crack-

ing. Unfortunately, the lower the crystallinity, the greater the likelihood

of permeation.

Resistance to stress cracking can be reduced by the absorption of sub-

stances that chemically resemble the polymer and will plasticize it. In addi-

tion, the mechanical strength will also be reduced. Halogenated chemicals,

particuarly those consisting of small molecules containing uorine or chlo-

rine, are especially likely to be similar to the uoropolymers and should be

tested for their effect.

The presence of contaminents in the uid may act as an accelerator. For

example, polypropylene can safely handle sulfuric or hydrochloric acids, but

iron or copper contamination in concentrated sulfuric or hydrochloric acid

can result in the stress cracking of polypropylene.

171

Corrosion of Linings

On many occasions, lining, coating, and paint terminologies are used inter-

changeably. Technically, linings and paints are both coatings. Usually, a coat-

ing is referred to as a lining when subjected to immersion, such as the interior

of a vessel, and paints refer to protective materials used to guard against

atmospheric corrosion. This chapter deals with coatings that are used to pro-

tect the interior of a vessel and will be subject to immersion.

Of the various coating applications, the most critical is that of a tank lining

application. The material must be resistant to the corrodent and be free of

pinholes through which corrosives might penetrate and reach the substrate.

The severe attack that many corrosives have on the bare tank emphasizes

the importance of using the correct material and the correct procedure in

lining a tank to obtain a perfect coating. It is also essential that the tank be

designed and constructed in the proper manner to permit the application of

a perfect lining.

In a tank lining there are usually four areas of contact with the stored

product that may lead to different types of corrosion. These areas are the

vapor phase (the area above the liquid level), the interphase (the area where

the vapor phase meets the liquid phase), the area always immersed, and the

bottom of the tank (where moisture and other contaminents of greater den-

sity may settle). Each of these areas can, at one time or another, be more

severely attacked than the rest, depending on the type of material contained,

the impurities present, and the amounts of oxygen and water present. In

view of this, it is necessary to understand the corrosion resistance of the coat-

ing material under consideration, and not just the immersed condition.

Other factors that have an effect on the performance of the coating mate-

rial are vessel design, vessel, preparation prior to coating, application tech-

niques of the coating material, curing of the coating, inspection, operating

instructions, and temperature limitations. In general, the criteria for tank

linings are:

1. Design of the vessel

2. Lining selection

3. Shell construction

4. Shell preparation

5. Lining application

6. Cure of the lining material

172 Fundamentals of Corrosion

7. Inspection of the lining

8. Safety

9. Causes of failure

10. Operating instructions

6.1 Liquid Applied Linings

Liquid applied linings are coatings that can be applied by spraying or by

troweling. As mentioned previously, if a tank lining is going to provide the

corrosion protection desired, it is essential that the:

1. Vessel is properly designed

2. Correct coating is specied for the application

3. Vessel shell is properly prepared

4. Coating is properly applied and inspected

6.1.1 Design of the Vessel

All vessels to be lined should be of welded construction. Riveted tanks will

expand or contract, thus damaging the lining and causing leakage. Butt-

welding is preferred but lap welding may be used, providing a llet weld is

used and all sharp edges are ground smooth (see Figure 6.1). Butt welds need

not be ground ush but they must be ground to a smooth, rounded contour.

A good way to judge a weld is to run your nger over it. Sharp edges can be

detected easily. All weld spatter must be removed (see Figure 6.2).

Grind smooth

Weld

Inside of vessel

DO DON’T

Gap

Round corners

Continuous ﬁllet weld

FigurE 6.1

Butt welding preferred rather than lap welding or riveted construction.

Corrosion of Linings 173

Any sharp prominence may result in a spot where the lm thickness will

be inadequate and noncontinuous, thus causing premature failure.

If possible, avoid the use of bolted joints. Should it be necessary to use a

bolted joint, it should be made of corrosion-resistant materials and sealed

shut. The mating surface of steel surfaces should be gasketted. The lining

material should be applied prior to bolting.

Do not use construction that will result in the creation of pockets or crev-

ices that will not drain or that cannot be properly sandblasted and lined

(Figure 6.3).

All joints must be continuous and solid welded. All welds must be smooth

with no porosity, holes, high spots, lumps, or pockets (Figure 6.4).

DON’T

Weld ﬂux

Weld spatter

Clean

Grind smooth

FigurE 6.2

Remove all weld spatter and grind smooth.

Very bad

inaccessible void

Cone or dome head

Shell

Skip weld

2 Channels

back to back

Round

corners

Full seam weld

DON’TDO

FigurE 6.3

Avoid all pockets or crevices that cannot be properly sandblasted and lined.

174 Fundamentals of Corrosion

All sharp edges must be ground to a minimum of 1/8-in. radius

(Figure 6.5).

Outlets must be of anged or pad type rather than threaded. If pressure

requirements permit, use slip-on anges, as the inside diameter of the attach-

ing weld is readily available for radiusing and grinding. If pressure dictates

the use of weld neck anges, the inside diameter of the attaching weld is in

the throat of the nozzle. It is therefore more difcult to repair surface irregu-

larities such as weld undercutting by grinding (see Figure 6.6).

Stiffening members should be placed on the outside of the vessel rather

than on the inside (Figure 6.7).

Tanks larger than 25 ft in diameter may require three manways for work-

ing entrances. Usually, two are located at the bottom (180° apart) and one at

the top. The minimum opening is 20 in., but 30 in. is preferred.

On occasion, an alloy is used to replace the steel bottom of a vessel. Under

these conditions, galvanic corrosion will take place. If a lining is applied to

the steel and for several inches (usually 6 to 8 in.) onto the alloy, any discon-

tinuity in the lining will become anodic. Once corrosion starts, it progresses

rapidly because of the bare exposed alloy cathodic area. Without the lining,

galvanic corrosion would cause the steel to corrode at the weld area, but at

a much lower rate. The recommended practice therefore is to line the alloy

Grind smooth

DO DON’T

Undercut

Rough

Pin hole

FigurE 6.4

All joints must be continuous solid welded and ground smooth.

DON’T

Sharp corner1/8 in. radius

Weld

FigurE 6.5

Grind all sharp edges to a minimum 1/8-in. radius.

Corrosion of Linings 175

Sharp corners

Inside of vessel

Sharp corner

Slip-on ﬂange

Line completely

to bolt circle

Full ﬁllet weld.

Grind and radius.

Inside of vessel

Round these

corners

DO DON’T

Pad type

reads

2 in. min.

Weld

Round corners

Flanged outlet

2 in. I.D.

Weld

FigurE 6.6

Typical vessel outlets.

Inside of vessel

Skip weld

Inside of vessel

Angle stiﬀener

DON’TDO

FigurE 6.7

Stiffening members should be on the outside of the vessel.

176 Fundamentals of Corrosion

completely as well as the steel, thereby eliminating the possible occurrence

of a large-cathode-to-small-anode area (see Figure 6.8).

It is important that the processing liquor is not directed against the side

of the tank, but rather toward the center. Other appurtenances inside the

tank must be located for accessibility of lining. Heating elements should be

placed with a minimum clearance of 6 in. Bafes, agitator base plates, pipe,

ladders, and other items can either be lined in place or detached and lined

before installation. The use of complex shapes such as I-beams should be

avoided. Sharp angles should be ground smooth and they should be fully

welded. Spot welding or intermittent welding should not be permitted.

Gouges, hackles, deep scratches, slivered steel, or other surface aws should

be ground smooth.

Concrete tanks should be located above the water table. They require spe-

cial lining systems. Unless absolutely necessary, expansion joints should be

avoided. Small tanks do not normally require expansion joints. Larger tanks

can make use of a chemical-resistant joint such as polyvinyl chloride (PVC).

Any concrete curing compound used must be compatible with the lining

material or removed before application. Form joints must be made as smooth

as possible. Adequate steel reinforcement must be used in a strong, dense,

concrete mix to reduce movement and cracking. The lining manufacturer

should be consulted for special instructions. Concrete tanks should be lined

only by a licensed applicator.

6.1.2 Lining Selection

The primary function of a lining system is to protect the substrate. An equally

important consideration is product purity protection. The purity of the liq-

uid must not be contaminated with by-products of corrosion or leachate from

the coating itself. Selection of a lining material for a stationary storage tank

Alloy (cathode)

Steel

Pit (anode)

Protective lining

FigurE 6.8

Potential galvanic action.

Corrosion of Linings 177

dedicated to holding one product at more or less constant time and tempera-

ture conditions is relatively easy because such tanks present predictable con-

ditions for coating selection. Conversely, tanks that see intermittent storage

of a variety of chemicals or solvents, such as product carriers, present a more

difcult problem because the parameters of operation vary. Consideration

must be given to the effect of cumulative cargoes. In addition, abrasion resis-

tance must be considered if the product in the tank is changed regularly with

complete cleaning required between loadings. Workers and equipment will

abrade the lining.

When selecting a coating system, it is necessary to determine all the condi-

tions to which the lining will be subjected. The conditions to consider fall

into two categories:

1. Chemicals to which the lining will be exposed

2. Conditions of operation

As a result, it is necessary to verify that the chemical resistance and physical/

mechanical properties of the lining are suitable for the application.

To specify a lining material, it is necessary to know specically what is

being handled and under what conditions. The following information must

be known about the material being handled:

1. What are the primary chemicals being handled, and at what concen-

trations?

2. Are there any secondary chemicals? And if so, at what concen-

trations?

3. Are there any trace impurities or chemicals present? This is extremely

important because concentrations in the ppm range can cause seri-

ous problems.

4. Are there any solids present? And if so, what are the particle sizes

and concentrations?

5. Will there be any agitation?

6. What are the uid purity requirements?

7. What will be present in the vapor phase above the liquor?

The answers to these questions will narrow the selection to those coatings

that are compatible. Following are the more common lining materials and

their general area of application:

High-bake phenolics• : excellent resistance to acids, solvents, food prod-

ucts, beverages, and water; most widely used lining material but has

poor exibility compared to other lining materials.

178 Fundamentals of Corrosion

Modied air-dry phenolics (catalyst required):• nearly equal in resistance

to high-bake phenolics; may be formulated for excellent resistance to

alkalies, solvents, salt water, deionized water, fresh water, and mild

acids: excellent for dry products.

Modied PVC polyvinyl chloroacetals, air-cured:• excellent resistance to

strong mineral acids and water; most popular lining for water stor-

age tanks; used in water immersion service (potable and marine)

and beverage processing.

PVC plastisols:• acid resistant; must be heat cured.

Hypalon:• chemical salts.

Epoxy (amine catalyst):• good alkali resistance; fair to good resistance

to solvents, mild acids, and dry food product; nds application in

covered hopper car linings and nuclear containment facilities.

Epoxy polyamide:• good resistance to water and brines; used in storage

tanks and nuclear containment facilities; poor acid resistance and

fair alkali resistance.

Epoxy polyester:• good abrasion resistance; used for covered hopper

car linings; poor solvent resistance.

Epoxy coal tar:• excellent resistance to mild acids, mild alkalies, salt

water, and fresh water; poor solvent resistance; used for crude oil

storage tanks, sewage disposal plants, and water works.

Coal tar:• excellent water resistance; used for water tanks.

Asphalts:• good acid and water resistance.

Neoprene:• good acid and ame resistance; used for chemical process-

ing equipment.

Polysulde:• good water and solvent resistance.

Butyl rubber:• good water resistance.

Styrene-butadiene polymers:• nds application in food and beverage

processing and in the lining of concrete tanks.

Rubber latex:• excellent alkali resistance; nds application in caustic

tanks (50–73%), at 180 to 250°F (82 to 121°C).

Urethanes:• superior abrasion resistance; excellent resistance to strong

mineral acids and alkalis; fair solvent resistance; used to line dish-

washers and washing machines.

Vinyl ester:• excellent resistance to strong acids and better resistance

up to 350 to 400°F (193 to 204°C), depending on thickness.

Vinyl urethanes:• nds application in food processing, hopper cars,

and wood tanks.

Fluoropolymers:• high chemical resistance and re resistance; used in

scrubber service.