Post Treatments

Wet Storage Film Inhibitors. A white film (sometimes called white rust or wet storage stain) may appear on zinc surfaces during storage or shipment. The film is found on material with newly galvanized, bright surfaces and especially in such areas as crevices between closely packed sheets and angle bars. Wet storage film can form if the surfaces come into contact with condensate or rainwater and the moisture does not dry quickly. Zinc surfaces that have developed a normal protective layer of corrosion products are seldom attacked.

When zinc coatings corrode openly in air, zinc oxide and zinc hydroxide are normally formed. In the presence of atmospheric carbon dioxide, these compounds are transformed to basic zinc carbonate. If the supply of air to the surface of the zinc coating is restricted, as in a narrow crevice, then sufficient carbon dioxide is not supplied for the formation of the normal layer of zinc carbonate.

The layer of zinc oxide and zinc hydroxide is voluminous and porous and adheres loosely to the zinc surface. Consequently, it does not protect the zinc surface against oxygen in the water. Corrosion can therefore proceed as long as there is moisture left on the surfaces. When wet storage film occurs, the objects should be arranged so their surfaces dry rapidly. The attack ceases, and with a free supply of air to the surfaces, the normal protective layer of corrosion products forms. The white corrosion products gradually wash off, and the surface of the coating takes on the normal appearance of a hot dip galvanized, exposed object.

Because the corrosion products are very voluminous (about 500 times that of the zinc that has been consumed), any attack may appear serious. Usually, however, such an attack of wet storage film is of little or no importance to the durability of the corrosion protection.

Wet storage film is best avoided by preventing newly galvanized surfaces from coming into contact with rain or condensate water during storage and transport. Materials stored outdoors should be arranged so that water can easily run off the surfaces and so that all surfaces are well ventilated (Fig. 17).

Fig. 17 Galvanized materials stacked with spacers and on an incline to prevent the formation of wet storage film

Temporary protection against wet storage film is obtained by chromating or phosphating. Painting after galvanizing also provides effective protection. Acrylic films containing corrosion inhibitors can also be applied to prevent the formation of wet storage film.

Where the surface staining is light and smooth without growth of the zinc oxide layer as judged by lightly rubbing fingertips across the surface, the staining will gradually disappear and blend in with the surrounding zinc surface as a result of normal weathering in service. When the affected area will not be fully exposed in service or when it will be subject to a humid environment, wet storage film must be removed, even if it is superficial, to allow formation of the basic zinc carbonate film which normally contributes to the corrosion resistance of galvanized coatings.

Medium to heavy buildup of white corrosion product must be removed, otherwise the essential protective film of basic zinc carbonates cannot form in affected areas. Light deposits can be removed by brushing with a 2% solution of sodium or potassium dichromate with the addition of 0.1 vol% of concentrated sulfuric acid. This is applied with a stiff brush and left for about 30 s before thoroughly rinsing and drying.

Paint Over Galvanizing. Hot dip galvanized steel may need to be painted for the following reasons:

• Additional corrosion protection for exposure to aggressive environments is needed, especially if future maintenance will be difficult or if the zinc coating is thin, such as on sheet metal.

• Another color of coating is desired for aesthetic reasons, for warning purposes, or for camouflage.

• Protection against galvanic corrosion is needed because the hot dip galvanized steel is to be in contact with another metal such as copper.

Hot dip galvanizing combined with painting offers good corrosion protection, even in very aggressive environments. The durability of such a duplex system is 1.5 to 2.7 times that of the durability of either the painted bare steel or the zinc coating alone.

The zinc coating can be painted immediately after hot dip galvanizing or after some time of exposure (Table 5). In most cases, painting immediately after hot dip galvanizing is preferable, since the surfaces are least contaminated.

Regardless of whether the paint is applied to a fresh, bright coating surface or to an exposed surface with corrosion products, the surfaces must be cleaned carefully prior to painting. The paint on zinc surfaces is more sensitive in this respect than many other materials, because even small quantities of impurities on the surfaces can affect the adhesion of the paint film.

However, the surfaces of zinc coatings are often much easier to clean than steel surfaces. It is important that an appropriate cleaning procedure be used for the particular impurities present on the surfaces.

Exposed Matte Surfaces. When zinc coatings are exposed, the surface corrodes and is covered with corrosion products. The basic zinc carbonate that forms in clean air can be painted. This is the reason for the traditional recommendation to wait from 6 months to 1 year before painting hot dip galvanized objects.

Today, however, the air is seldom clean. The layer of corrosion products contains such substances as sulfides, sulfites, sulfates, and chlorides. Many of these compounds are water soluble and some are even hygroscopic. To achieve good results when painting, all water-soluble impurities must be removed.

Cleaning and Surface Preparation. Heavily contaminated surfaces, both fresh and exposed, should be washed with a suitable organic solvent such as white mineral spirits, and then bristle brushed to remove solid particles and corrosion products. This washing should be followed with a thorough rinsing with water at high pressure, if possible.

Moderately contaminated surfaces, for example, fresh newly galvanized surfaces and surfaces that have been exposed for a longer period of time but have not been contaminated with oil and grease, can be washed with water to which 5 to 10% ammonia, caustic soda (NaOH), or acetic acid has been added. Afterwards, the surface should be buffed with a soft brush. This treatment must be followed by very thorough rinsing with water at high pressure, if possible.

Chromated surfaces, on continuously hot dip galvanized sheet, for example, can also be washed with ammonia, caustic soda, or acetic acid in water and buffed, followed by thorough rinsing. The alkaline or acid solution dissolves the chromate layer. In general, when galvanized after fabrication material is to be painted as a post-treatment, it should not be chromate treated.

Surfaces that have been exposed, moderately contaminated surfaces, or newly galvanized surfaces can also be brush blasted, that is, blasted with low pressure and a rapid motion of the nozzle, for example 0.3 MPa (0.04 ksi) at 6 mm (0.2 in.) nozzle diam and 250 to 300 mm (9.8 to 11.9 in.) nozzle distance. Abrasives consisting of silicates and slags of 0.2 to 0.5 mm (0.008 to 0.02 in.) are recommended. Glass beads and fine-grained aluminum oxide can also be used.

Sweep blasting effectively removes any corrosion products and provides an advantageous roughening of the surface of newly applied bright zinc coatings. However, brush blasting must be carried out carefully so that the zinc coating is not destroyed and large stresses are not built into the coating. These stresses may subsequently cause flaking of the paint coat.

General Surface Conditions After Galvanizing. Table 8 provides a general guide to the inspection of galvanized surfaces.

Table 8 Guide for visual inspection of galvanized surfaces

Condition

Causes

Grounds for rejection?

Bare spots

Paint, grease or oil residues

Yes, except where bare spots are small and suitable for patching

Scale or rust residues

Residual welding slag

Rolling defects in basis steel

Embedded sand in castings

Overdrying of preflux

Excess aluminum in bath

Articles in contact during galvanizing

General roughness

Analysis or original surface condition of steel

No, except by prior agreement

Overpickling

Uneven cold working

High galvanizing temperature and/or long immersion time

Dross protrusions

Entrapped dross particles

No, unless dross contamination is heavy

Blisters

Surface defects in steel

No

Absorbed hydrogen

Not if due to steel composition

Lumpiness and runs

Withdrawal speed too high

Only on basis of prior agreement

"Cold" galvanizing bath

Delayed run-off from seams,joints, bolt holes, etc.

Articles in contact during withdrawal

Flux inclusions

Stale flux burnt on during dipping

Yes

Surface residues on steel

Yes

Flux picked up from top of bath

Yes, unless removed

Ash inclusions

Ash burnt on during dipping

Yes, if in gross lumps

Ash picked up from top of bath

Dull gray coating or mottled appearance

Steel composition (high silicon, phosphorus, or carbon) or severe cold work

Not if due to steel composition or condition, or limited to occasional areas

Slow cooling after galvanizing

Rust stains

"Weeping" of acid, etc., from seams and folds

No

Storage on or near rusty material

No

Wet storage film ("white rust")

Confinement of close-packed articles under damp conditions

No, unless present prior to first shipment or unless severely pitted.

Packing of articles while damp

Customer to exercise caution during transportation and storage

Choice of Paint. Paints suitable for direct application to properly cleaned hot dip galvanized steel are discussed below. As with most other paints, first apply a suitable primer to the zinc surface.

Paints consist of 10 to 20 different components and each different manufacturer has its own formula for a certain type of paint. Paints of the same type but from different manufacturers can have different properties. Detailed recommendations can be obtained from the manufacturer. Table 9 compares characteristics of some of the most common paints used with galvanized coatings.

Table 9 Comparative characteristics of paints and paint films used on hot dip galvanized steel

E, excellent; G, good; F, fair; P, poor. Other symbols used are defined in the table footnote.

Table 9 Comparative characteristics of paints and paint films used on hot dip galvanized steel

E, excellent; G, good; F, fair; P, poor. Other symbols used are defined in the table footnote.

Paint (film) characteristics

Numbers of paints in Table 5

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Application method®

B

B

B

S

S

S/B

S/B

S

S

S

S/B

S

S

S

S

Drying time®

Lo

Lo

Mi

Sh

Sh

Mi

Mi

Mi

Sh

Sh

Mi

Mi

Mi

Mi

Mi

Hardening-through time®

Lo

Lo

Mi

Mi

Mi

Lo

Lo

Mi

Mi

Mi

Lo

Mi

Mi

Lo

Mi

Hardness

F

F

F

G

G

F

F/G

G/E

G

G

F

G

F/G

F

G/E

Flexibility

E

E

E

F/G

G

G

G

G

G

G

G

F/G

F/G

G

G

Impact resistance

E

E

E

G

E

G

G/E

E

G

E

G

G

G

G

E

Gloss retention®

G

E

E

P

F

F

F

E

P

F

F

P

F

P

E

Color retention1"6"1

P

F

G

P

F

E

E

E

P

F

F

F

F

E

Can stability®

G

G

E

E

E

G

G

P

E

E

G

P

G

P

P

Thermal resistance®

P

P

F

P

P

F

F

G

P

F

F

F

F

P

G

Weather resistance®

Rural

E

E

E

E

E

E

E

E

E

E

E

E

E

P

E

Marine

G

G

G

E

E

F

E

E

E

E

G

E

G

G

E

Industrial

G

F

G

G

E

F

F

G

G

E

G

E

F

E

G

Resistance®

Acid solutions

F

P

F

E

E

P

F

G

G

G

F

E

F

E

G

Alkaline solutions

P

P

P

E

E

P

P

F

G

G

P

G

P

E

E

Note: Paint and film characteristics may show differences because of variations in paint formulations; therefore, all indications are relative.

Note: Paint and film characteristics may show differences because of variations in paint formulations; therefore, all indications are relative.

Source: Ref 3

(a) B, mainly by brush; S, mainly by spraying (air or airless).

(b) Drying time to tack-free: Sh, short (<1 h); Mi, intermediate (1 to 4 h); Lo, long (4 to 12 hours).

(c) Hardening-through time: Sh, short (3 h); Mi, intermediate (1 to 7 days); Lo, long (>7 days).

(d) Poor and fair ratings are mainly due to chalking.

(e) Poor and fair ratings are mainly due to yellowing.

(f) E, stable for more than 6 months; G, stable for approximately 6 months; F, stable for approximately 1 month; P, stable for less than 36 h (after adding hardeners).

(g) E, permanent resistance to temperatures between approximately 50 and 150 °C (120 and 300 °F); G, resistance to temperatures between approximately 50 and 75 °C (120 and 165 °F); F, resistance to temperatures between approximately 50 and 75 °C (120 and 165 °F) for short periods only; P, practically no thermal resistance above 50 °C (120 °F).

(h) Not counting changes in gloss and/or color.

(i) Indications depend on time of exposure and concentration and temperature of aqueous solutions.

In moderately corrosive atmospheres paints based on acrylate and PVAc-latex are suitable. However, it takes about 10 to 14 days for these paints to achieve maximum hardness and adhesion. If the objects are to be handled or transported within this time, special care must be observed to avoid damage.

Under severe chemical conditions, such as in industry, and in aggressive atmospheres, paints with better chemical resistance than latex paints are required. Such paints are based on PVC, vinyl copolymers, chlorinated rubber, polyurethane, and epoxy.

In water and soil, tar/bitumen paints are recommended, preferably in combination with epoxy. and polyurethane. Certain aluminum-pigmented asphalt solutions can also be used for structures in water, but they have relatively poor mechanical strength.

Additional information is available in the article "Painting" in this Volume.

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