Sacrificial Coatings

7.4.3 Sacrificial Coatings

Figure 7.8 illustrates in a simple fashion the concept of a sacrificial coating. Sacrificial coatings use a metal that is anodic to steel and which corrodes preferentially. Essen- tially, sacrificial coatings provide cathodic protection, especially in the vicinity of film defects. Sacrificial coatings:

Figure 7.9 Illustration of Adhesion Concept

• Usually contain zinc dust as the predomi-

Adhesion can be chemical, mechanical,

nant pigment

polar, or a combination of all three. The

• Must have a minimum loading of zinc

chemical bond, formed by a reaction

between the coating and the substrate is the Examples of sacrificial coatings include

dust to be effective

most effective bond. An example of a chem- inorganic zinc and flame-sprayed aluminum

ical bond is the galvanizing process, in (FSA).

which the molten zinc melts the surface

Coating Fundamentals 7-7

layer of the steel and the two materials com-

7.6 Basic Inspection

bine and form a series of alloys, essentially

Considerations

making the coating a part of the surface. The coating inspector should check to see if Inorganic zinc coatings also form a chemical

the materials delivered to the job site (coat- bond between the silicate molecule and the

ings, thinners, cleaners, abrasives, putties, steel substrate. Wash primers that normally

fillers, etc.) are the materials shown in the include an acid element also form a chemi-

specification or as approved by the owner or cal bond with the substrate.

his representative.

Polar adhesion (aka valance bonding) is The inspector should have a copy of each the most common type of bond for organic

approved product data sheet to compare the coatings. The resin acts as a weak magnet

labels on the cans with the approved mate- with the north and south poles attracting

rial named on the product data sheet. Any opposite poles on the substrate. Another

substitution of material should be reported explanation of this is that the polar groups

immediately to the owner’s representative. are positively and negatively charged por-

Be aware that even when two products have tions of the coating molecule that are

the same generic name they are not neces-

attracted to oppositely charged areas on the

sarily equal or even equivalent.

substrate. Epoxies fall within this type of The inspector should also check: adhesion concept.

• Shelf life

Mechanical adhesion is associated with

• Storage conditions and temperatures

surface roughness (anchor pattern). The

• Batch numbers and record them for each

roughness of the surface created by some

unit of coating

types of surface preparation allows more points of contact between the molecules of

Batch numbers are always found on the the coating and the molecules of the surface.

coating pail, not on the product data sheet. It As contact points increase, adhesion

may be necessary to check each can in each increases.

shipment to ensure it is the proper material and to record all the batch numbers.

Concrete has a different type of mechanical adhesion. The concrete surface is relatively porous with many minute surface pockets,

It is not unusual for a surface checking, and other natural surface

manufacturer to mix products on roughness. Coatings for such surfaces

a pallet or to ship the same should be highly penetrating.

product but from different batches on the same pallet.

All forms of adhesion depend on direct con- tact between the coating and the substrate.

When two component materials are being Any type of contaminant on the surface

used, the inspector should ensure that the interferes with this point-to-point contact

correct amount of each component is on site, and reduces the amount of adhesion.

based on the mix ratio.

7-8 Coating Fundamentals