Hdg & Hsg Insp & Repair - Aga

AMERICAN GALVANIZERS ASSOCIATION

The Inspection of

Products to be

Hot-Dip

Galvanized

After Fabrication

Including a New

Section on

Touch-Up and Repair


THE INSPECTION OF PRODUCTS HOT-DIP GALVANIZEDAFTER FABRICATIONTABLE OF CONTENTS

Purpose of Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1Sampling for Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1Coating Microstructure and Causes of Thickness and Uniformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2Thickness Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

Magnetic Thickness Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5Stripping and Weighing Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5Weighing Before and After Galvanizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Thickness Testing Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6Magnetic Balance Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6Pull-off Magnetic Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6Electronic Thickness Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Coating Adherence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7Testing for Adherence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8Appearance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8Pre-galvanizing Consultation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9Visual Inspection Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Bare Spots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9General Roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10Dross Protrusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10Lumpiness and Runs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10Flux Inclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10Ash Inclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11Matte Gray or Mottled Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11Rust Stains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11Wet Storage Stain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Miscellaneous Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12Testing for Chromate Finishes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12Embrittlement Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

Touch-up and Repair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13When is Touch-up Necessary? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Touch-up and Repair Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

Metallizing (Zinc Spraying) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14Performance Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

2001 American Galvanizers Association. The material in this publication has been developed to provide accurate and authoritative information about the inspection ofproducts hot-dip galvanized after fabrication. This material provides general information only and is not intended as a substitute for competent professional examination andverification as to suitability and applicability. The publication of the material herein is not intended as a representation or warranty on the part of the American GalvanizersAssociation, Inc. Anyone making use of this information assumes all liability arising from such use.


Zinc-rich Paint Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15Performance Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

Soldering with Zinc-based Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16Performance Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

Repair Method Selection Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17Related Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

TABLE OF CONTENTS CONT.

1AMERICAN GALVANIZERS ASSOCIATION

PURPOSE OF INSPECTIONHot-dip galvanizing after fabrication is one of the most widely used meth-

ods of corrosion protection. The final step in the hot-dip galvanizing processis inspection to ensure compliance with specifications. Interpretation ofinspection results must be made with a clear understanding of the causes ofvarious coating conditions and their affects on the ultimate objective of pro-viding corrosion protection.

First and foremost, the purpose of hot-dip galvanizing is to protectsteel from corrosion. The length of time this protection can reasonably beexpected to last before minimal maintenance is required is called its servicelife. Galvanizings service life is directly related to the thickness of the pro-tective zinc coating: the thicker the coating the longer the service life (seeFigure 1). Thus, coating thickness is the single most important inspectioncheck to determine the quality of a galvanized coating.

Coating thickness, however, is only one item of inspection. Coating uni-formity, adherence and appearance also are evaluated. Additionally, embrit-tlement and other defects arising from fabrication and design are inspectionconcerns.

While minimum standards must be satisfied in all these areas, their rel-ative importance varies according to the end-use of the finished product. Forexample, the end-use requirement for galvanized structural steel in an iso-lated area differs from that for a thin-gauge product used in a decorativeapplication. Understanding the individual requirements of the product andthe capability of the hot-dip galvanizing process is essential for properinspection.

Inspection of the galvanized product is most effectively and efficientlyconducted at the galvanizers plant. Here, questions and concerns can beraised and dealt with quickly and efficiently - speeding up the inspectionprocess and resulting in a time savings that is an asset to the overall project.

SAMPLING FOR TESTSTo properly evaluate hot-dip galvanized coat-

ings, it is essential that selected specimens be rep-resentative of the inspection lot. Because inspec-tion lot sizes can be very small, statistical samplingplans, such as covered in American Society ofTesting & Materials (ASTM) specification B 602and Canadian Standards Association (CSA) speci-fication Z 90, may not be applicable. However,such a statistical sampling plan is recommended forlarger lots.

The inspection lot is a collection of galvanizedarticles of the same kind that: were galvanized at approximately the same time, were galvanized in the same manner, were galvanized in the same galvanizing kettle,

and are being submitted for acceptance as a group.

After items are removedfrom the galvanizing bath,

the inspection processbegins

Service-Life Chart for Hot-Dip Galvanized CoatingsDerived from The Zinc Coating Life Predictor (fortjava.com:8080/zclp/index.html)

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7 8

Average Zinc Thickness (microns top line, mils bottom line)*Service Life is defined as the time to 5% rusting of the steel surface

Serv

ice

Life

* (yr

s.)

25 50 75 100 125 150 175 200

Note: 1 mil ~ 1.8 oz./ft 2

3.9 mils of zinc coating is the minimum thickness for 1/4" thick structural steel, as governed by ASTM A 123-01, Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products

Figure 1

Rural

Suburb

an

Tempera

te Marine

Moderate

ly Indust

rial

Tropical Mar

ine

2 AMERICAN GALVANIZERS ASSOCIATION

ASTM A 123, section 7, provides guidelines for the number of speci-mens that need to be tested per lot. For small objects such as nuts, bolts andwashers, an entire article should be the test specimen.

For large articles such as plates, bars and angle sections, tests should beconducted on the galvanized article according to the procedure described inASTM A 123, section 6.1. The measurement of coating thickness should betaken at widely dispersed points to represent a true sampling of the wholepart. Extremely large parts may need to be tested in sections in order to rep-resent the entire product.

COATING MICROSTRUCTURE AND THE CAUSES OF THICKNESSAND UNIFORMITY

The thickness of the galvanized coating is the primaryfactor in determining its service life. The thicker the coating,the longer the corrosion protection lasts.

The factors affecting coating thickness are a combina-tion of several variables, some of which the galvanizer cancontrol and some of which are beyond the galvanizers con-trol. The chemical composition of the steel plays the biggestrole in determining the thickness of a galvanized coating.

During the galvanizing process, a complex metallurgicalreaction takes place creating a series of zinc-iron alloy lay-ers. These layers contain varying amounts of zinc and iron,depending on the proximity to the base steel. The layersclosest to the base steel contain more iron and less zinc thanthe layers farther away. Figure 2 depicts the zinc-iron alloylayers as seen in a typical after-fabrication, hot-dip galva-nized coating. Normally, the alloy-layer growth tapers off asthe steel reaches the bath temperature. As the item isremoved from the bath, a free zinc layer forms, giving thegalvanized coating its familiar shiny, silver appearance.

However, certain steel compositions tend to acceleratethe growth of the zinc-iron alloy layers so that the galva-nized coating has a matte finish with little or no pure zincouter layer (see Figure 3). Steel containing carbon in excessof 0.25 percent, phosphorus in excess of 0.04 percent, ormanganese in excess of about 1.35 percent have been shown

to create these heavy coatings. This coating also tends to be thicker than tra-ditional bright, shiny galvanized coatings. The galvanizer has no controlover this reactive silicon-killed steel condition, which is illustrated in Figure4. These thicker coatings frequently have a dark gray, matte appearance dueto the lack of a free zinc layer capping the alloy layers.

A silicon level greater than 0.30 percent is particularly influential in pro-ducing heavier zinc-iron alloy layer coatings.

The surface condition of the steel also influences the thickness andsmoothness of the galvanized coating. Non-reactive steels that have beenabrasively cleaned prior to galvanizing can have coating thicknesses 50 to100 percent greater than steels only chemically cleaned.

In reactive steels, the opposite effect can be seen with steels that havebeen abrasively cleaned. The coating thickness on these steels is generally

Eta (100% Zn)

Zeta (94% Zn 6% Fe)

Delta (90% Zn 10% Fe)

Gamma (75% Zn 25% Fe)

Steel

1

2

2

4

3

4 6

5

6

7

8 10 12

Coa

ting

Wei

ght

(oz/

ft2 )

Killed Steel

Unkilled Steel

Figure 2. Typical zinc/ironalloy layers

Figure 3. Irregular zinc/ironalloy layers

Figure 4. Coating weights onreactive steels

Free Zinc Layer

Zinc-Iron AlloyLayers

Steel


lower than expected. The appearance, however, remains a matte gray withoccasional roughness.

The mass, the shape and the amount of cold working of the piece alsoaffect coating thickness and uniformity. When a fabricated article has bothheavy and light sections, differences in coating thickness between the sec-tions may result. Since immersion time varies according to the relationshipof the surface area of an item to its weight, the galvanizer has little controlover this situation.

Variables the galvanizer can control are bath temperature and withdraw-al rate. Because formation of the zinc-iron alloy layers is a diffusion process,higher bath temperatures generally produce heavier alloy layers. Like manydiffusion processes, the reaction proceeds rapidly at first and slows down asthe alloy layers become thicker.

The thickness of the outer zinc layer largely depends on the rate of with-drawal from the zinc bath and the drain-off of excess zinc. A faster rate ofwithdrawal causes an article to carry out more zinc. This results in a heaviercoating.

When hot-dip galvanizing fabricated articles, local differences in thedrain-off, because of the shape of the article and the angle at which differentsurfaces leave the bath, will generally result in some variation in coatingthickness. Specifications for hot-dip galvanizing recognize that variations incoating thickness are inherent in the process. The minimum thickness of thezinc coating is always specified as an average thickness of specimens testedand a minimum weight for any individual specimen.

When measurements are made to determine the thickness distribution ofa large galvanized article, a sufficient number of readings, not less than fiveand preferably 10, should be taken at each end and in the middle of the arti-cle being examined (see Figure 5). The measurements in each area shouldbe taken at least four inches from the edge to avoid end effects and as wide-ly dispersed as possible. Usually, the end of an arti-cle that leaves the bath last will carry a thicker coat-ing. This is particularly so towards the edge, whereat the time of drainage the last few drops of the zinctend to collect as a result of surface tension.

The minimum coating requirements specifiedby the ASTM for different classes of work are sum-marized in Table 1 for ASTM A 123, Table 2 forASTM A 153 and Table 3 for ASTM A 767. Thethickness of a zinc coating is reported in thou-sandths of an inch. For ease of conversion to thick-ness in inches and mils, one ounce of zinc persquare foot is equivalent to 0.0018 inches or 1.8mils. A conversion chart for common coatingweights in both English and metric units is provided in Table 5.

The CSA minimum coating requirements for different classes of workare summarized in Table 4.

THICKNESS TESTINGThere are several methods to determine the weight or thickness of the

zinc coating on a galvanized article. The methods of testing chosen will mostlikely be dictated by the size, shape and number of pieces to be tested. Some

Figure 5. Measurements should be taken at each endand in the middle of the piece

Appearance of reactive andnon-reactive steels after

galvanizing


Minimum Average Coating Thickness Grade by Material Category - ASTM A 123(rolled, pressed and forged shapes, castings, plates, bars and strips)

Material Category All Specimens TestedSteel Thickness Range (Measured), in. (mm)

4.8 to 1/4 (>6.4)

Structural Shapes 45 65 75 85 100Strip and Bar 45 65 75 85 100Pipe and Tubing 45 45 75 75 75Wire 35 50 60 65 80

Minimum Coating Thickness by Class -ASTM A 767 (reinforcing bars)

Mass of Zinc CoatingCoating Class min., g/m2 of Surface

Class I 1070Class II 610

Coating Class Weight of Zinc Coatingmin., oz/ft2 of Surface

Class IBar designation size no. 3 3.00Bar designation size no. 4 and larger 3.50

Class IIBar designation size no. 3 and larger 2.00

Minimum Average Coating Thickness by Material Class - ASTM A 153 (iron and steel hardware)

Class of Material Minimum Weight of Zinc Coating, oz/ft2 (g/m2) of SurfaceA

Average of Specimens Tested Any Individual Specimen

Class A - Castings, Malleable Iron, Steel 2.00 (610) 1.80 (550)Class B - Rolled, pressed and forged articles (except

those which would be included under Class C or D)B-1 - 3/16 in. (4.76 mm) and over in thickness and over

15 in. (381 mm) in length 2.00 (610) 1.80 (550)B-2 - Under 3/16 in. (4.76 mm) in thickness and over 15 in.

(381 mm) in length 1.50 (458) 1.25 (381)B-3 - Any thickness and 15 in. (4.76 mm) and under in length 1.30 (397) 1.10 (336)

Class C - Fasteners over 3/8 in. (9.52 mm) in diameter and similararticles. Washers 3/16 in. and 1/4 in. (4.76 and 6.35 mm)in thickness 1.25 (381) 1.00 (305)

Class D - Fasteners 3/8 in. (9.52 mm) and under in diameter, rivets,nails and similar articles. Washers under 3/16 in. (4.76 mm)in thickness 1.00 (305) 0.85 (259)

A In the case of long pieces, such as anchor rods and similar articles over 5 ft (1.52 mm) in length, the weight of coating shallbe determined at each end and the middle of the article. In no case shall individual measurements be below the minimumshown in the Any Individual Specimen column.

Table 1

Table 4

Table 2

Minimum Mass of Zinc Coatings - CSA G 164

Classification Minimum mass Equivalentof Material of zinc coating minimum

g/m2 (oz/ft2) thicknessum (mils)

Castings, Iron and Steel 550 (1.80) 78 (3.00)Rolled, drawn, pressed or forged steel articles1 mm (0.039 in) and up but not

including 2 mm (0.079 in) 260 (0.85) 37 (1.44)2 mm (0.079 in) and up but not



including 5 mm (0.196 in). 560 (1.84) 80 (3.13)5 mm (0.196 in) thick and heavier 610 (2.00) 87 (3.40)

Refer to CSA G 164 for complete minimum coating thicknesses,including fasteners and similar objects.

Table 3


test methods are non-destructive; othermethods are destructive, since theyrequire the removal of the zinc coat-ing or sectioning of the coated mater-ial.

Magnetic Thickness Measurements - The thickness of thecoating may be determined by mag-netic thickness gauge measurementsin accordance with ASTM E 376. Aminimum of five readings shall betaken for each specimen. The averageof the thickness values taken for eachspecimen shall be not less than one coating thickness grade lower than thevalue listed in the appropriate specification. If these coating thickness mea-surements are made on an article with different thicknesses of steel, the val-ues in the appropriate specification apply to each thickness of steel on thearticle.

Stripping and Weighing Method - The average weight of a zinc coatingmay be determined by stripping an entire piece in accordance with ASTMA 90; alternatively, the average coating weight may be determined by strip-ping test pieces from the representative part, each with a measurable area ofcoated surface of at least 10 in.2 (64.5 cm.2). Extract one specimen approxi-mately 4 in. (100mm) from each end of the member and a third specimenfrom the approximate center of the member. The weight of coating obtainedat each location shall not be less than the value listed in the appropriate spec-ification. The average weight of coating for the item shall be the average ofthe values obtained in the three locations and shall not be less than the valuelisted in the appropriate specification. If these coating weight measurementsare made on an article with different thicknesses of steel, then the values inthe appropriate specification shall apply to each thickness of steel on thearticle.

Weighing Before and After Galvanizing - The average weight of thezinc coating may be determined by weighing the articles before and aftergalvanizing, subtracting the first weight from the second, and dividing theresult by the determined surface area. The first weight shall be determinedafter pickling and drying, the second after cooling to ambient temperature.

The weighing before and after galvanizing method does not take intoaccount the weight of iron reacted from the article that is incorporated intothe coating. It may thus underestimate coating weight by as much as 10 per-cent. Steel reactivity will affect the extent of underestimation.

Microscopy - The thickness of the coating may be determined by cross-sectional and optical measurement in accordance with ASTM B 487. Thismethod requires the use of an optical microscope with a calibrated eyepiece.The thickness determined by this method is a point value. No less than fivesuch measurements shall be made at locations on the test article that are aswidely dispersed as practical so as to be representative of the whole surfaceof the test article. The average of no less than five such measurements is thespecimen coating thickness. The microscopy method is a destructive test andmay be appropriate for smaller articles, but would not be practical for larg-er articles.

Testing coating thickness byusing magnetic measuring

devices

Weighing the article beforeand after galvanizing

Coating Conversion Chart

Coating Grade mils oz/ft2 um g/m2

35 1.4 0.8 35 24545 1.8 1.0 45 32055 2.2 1.3 55 39065 2.6 1.5 65 46075 3.0 1.7 75 53085 3.3 2.0 85 600100 3.9 2.3 100 705

Table 5


THICKNESS TESTING GAUGESThere are a number of simple magnetic gauges that can be used to give

a quick and convenient measurement of the zinc coating thickness. Thesemagnetic gauges give reliable thickness readings provided they are properlycalibrated against non-magnetic coatings of known thickness and the manu-facturers instructions are observed.

The most commonly used magnetic gauges provide readings based onmagnetic attraction between the gauge and the base steel. Two instrumentsof this type are magnetic balance gauges and pull-off gauges.

Magnetic Balance Gauges are based upon the variation in the force ofattraction between two ferromagnetic bodies as a function of the distancebetween them. Coating thickness measurements are taken by placing the

rubber magnet housing on the coatings surface with the gauge heldparallel to the surface. The scale ring of the gauge is turned forwardto bring the magnetic tip into contact with and perpendicular to thecoating surface. The scale ring is slowly turned by hand until thespring tension on the magnet just overcomes the attractive forcebetween the magnet and substrate of the galvanized item. At thispoint, the magnetic tip breaks loose from the coated surface and theinspector stops rotating the scale ring. The break in contact is shownby an indicator and can also be heard and felt. Since the strength ofthe magnetic attraction relates to the thickness of the coatingbetween the magnet and substrate, the spring tension required toequal this force is measurable. The scale ring has been calibrated inunits of coating thickness (mils or microns).

This type of gauge has the advantage of being able to measurecoating thickness in any position, without recalibration and withoutinterference from gravity, because the pivot arm is balanced. Thetypical accuracy of the magnetic balance type gauges is plus or

minus 10 percent of the indicated readings.Pull-off Magnetic Gauges are also based on magnetic attraction to the

underlying steel. They are primarily intended for use in the field as a roughguide to determine if the coating is within the thickness specification.

To take a measurement with a pull-off gauge, the magnet end of thegauge is placed vertically on the surface of the coating. The gauge is drawnaway, thus extending a spring. The reading is taken on the scale at the low-est point where the magnet breaks from the coated surface. Before each use,the hemispherically tipped magnet should be carefully inspected for dirt,small steel particles, tacky paint film and tip wear. Wear on the magnet willalter the calibration of the gauge. The typical accuracy of a pull-off gauge isplus or minus 15 percent, provided the gauge is used within a true verticalplane. If the gauge is used in a horizontal or overhead position, more errorwill be inherent.

Besides accuracy limitations, the pull-off type gauge has other disad-vantages: the inspectors eye must record the coating thickness as the mag-net breaks away from the coating, and erroneous readings will result if themagnet is allowed to slide over the coating prior to breakaway.

Another type of gauge is an Electronic Thickness Gauge. This uses atemperature-compensated magnetic transducer to measure the magnetic fluxchanges that occur when the probe (a magnet) is separated from the coatedferrous substrate. The output signal from the probe is proportional to the dis-Pull-off gauge

Magnetic balance gauge


tance of separation and, therefore, to coating thickness. The probe signal isamplified and displayed to show coating thickness. These battery-poweredinstruments have typical accuracies of plus or minus 5 percent and have theadvantage of not requiring recalibration with probe orientation.

To avoid possible sources of error in the use of magnetic instruments,certain precautions should be taken: Follow the manufacturers instructions. The instrument should be fre-

quently recalibrated against non-magnetic film standards of knownthickness.

Never expose any magnetic gauge to strong AC or DC fields because themagnet will change, affecting the calibration of the gauge.

The base steel should be backed up with similar material if thinner thanthe critical thickness for the magnetic gauge, or the instrument should berecalibrated on a substrate of similar thickness.

Readings should not be taken near an edge, hole or inside corner. Readings should not be taken on curved surfaces without proper

recalibration. The test surface should be free from dirt, grease, oxides and corro-

sion products. Test points should be chosen to avoid obvious peaks or irregulari-

ties in the coating. A sufficient number of readings should be taken to obtain a true

average.ASTM E 376 and CSA G 164-M provide guidance for use of these

instruments and factors affecting their accuracy.Metallographic Examination

Where the galvanized coating microstructure and thickness are ofinterest, microscopic examination is a reliable tool. This very accuratemethod uses a small polished and etched cross-section specimen of thegalvanized work to provide information about the relative thicknesses of thealloy and the free zinc layers that comprise the galvanized coating.Important disadvantages of this technique are that specimens are required tobe cut from the galvanized article, coating thickness provided only appliesto a very limited area, it does not indicate the variation in coating distribu-tion on the article, and it is necessary to examine a number of specimens todetermine the average coating thickness on the galvanized article.

COATING ADHERENCEThe hot-dip galvanized coating should be sufficiently adherent to with-

stand handling consistent with the nature and the thickness of the coating innormal use of the article, without peeling or flaking. Bending or forming,other than straightening after hot-dip galvanizing, are not considered to benormal handling.

When certain grades of steels or very heavy steel sections are galva-nized, thicker-than-normal coatings may occur. The galvanizer has littlecontrol over this because these thicker coatings are a function of the steelschemical composition or the longer immersion time required for massiveitems. Heavy galvanized coatings are more brittle than thin coatings; conse-

Taking field readings with anelectronic thickness gauge

Electronic thickness gauge


quently, application and interpretation ofthe standard adherence test must take thisinto account.

TESTING FOR ADHERENCEOne method is recognized for testing

galvanized coatings for adhesion: the stoutknife test. While it is not a true measure ofthe adhesive strength of the metallurgicalbonding of the galvanized coating to thebase steel, it serves as an indicator of theadherence properties of the coating.

This simple but effective test is con-ducted by prying the galvanized coatingwith the tip of a sharp knife. Considerable

pressure is exerted in a manner tending to remove a portion of the galva-nized coating. If the coating flakes off or disbonds in advance of the knifepoint it is non-adherent. If the coating separates at the point it is adherent.Removal of small particles of coating is not considered failure. This test isdetailed in ASTM A 123 and A 153, and in CSA G 164-M.

APPEARANCEThe ability of a galvanized coating to meet its primary objective of pro-

viding corrosion protection should be the chief criteria in evaluating its over-all appearance and in determining its suitability. The basic finish require-ments of the galvanized coating are that it be relatively smooth, continuousand free from gross surface imperfections. Smoothness is an ambiguousterm; the products end use must be the determining factor in setting toler-ances for smoothness. The galvanized coating should be continuous to pro-

vide optimum corrosion protection. Handling techniques for galvanizing may require the use of

chain slings, wire or other holding devices to lower material into thegalvanizing kettle, if suitable lifting features are not available on theitem. Chains, wires and special jigs used to handle the items mayleave a mark on the galvanized item. These marks are not necessar-ily detrimental to the coating, nor are they cause for rejection unlessthey have exposed the base metal. If considered necessary, theseareas can be easily touched up using the procedures described inASTM A 780.

Differences in the luster and color of galvanized coatings do notsignificantly affect corrosion resistance. The presence or absence ofspangle has no affect on coating performance. The well-knownspangled effect found on galvanized products is a crystallizationprocess that is dependent upon the zinc bath chemistry, the rate ofcooling, the method of pickling, the steel chemistry, and the thick-ness of the coating. Dull gray or patchy matte gray galvanized coat-ings give a service life equal to bright or spangled coatings since theservice life depends on the zinc coating thickness. Variations in

coating appearance or finish are important only to the extent that they willaffect corrosion performance or the intended use of the article. The primaryfunction of the galvanized coating is corrosion protection.

Testing coating adherence

Flaking

Superficial marks left on the galvanizedcoating from the chains used in theprocess are not grounds for rejection


PRE-GALVANIZING CONSULTATIONHigh quality galvanizing begins on the drawing board. Whenever a

question arises on the advisability of galvanizing a certain weld material,fabrication or steel type, consult the galvanizer. The AGA publication TheDesign of Products to be Hot-Dip Galvanized After Fabrication describes indetail proven methods of achieving a high quality galvanized coating before galvanizing.

Consultation among the designer, fabricator and galvanizer is desirableat all stages in the design and fabrication process. Most of the issues definedand explained in this publication can be avoided by the involved parties dis-cussing the product before it arrives at the galvanizing plant.

VISUAL INSPECTION GUIDEBare Spots

Because of zincs sacrificial action, small localized flaws are somewhatself-healing and have little effect on the service life of the coating. Whereconsidered necessary, such spots may be repaired using one of the repairmethods indicated in ASTM A 780. Any unrepairable, uncoated areasshould be rejected. Some of the causes of bare spots on galvanized steel aredescribed below.

Inadequate Surface Preparation - Thorough preparation of the steel isthe foundation of good galvanizing. Remnants of paint, oil, grease, scale, orrust are the most common causes of uncoated spots. Such residues are notwetted by the molten zinc and, therefore, prevent normal coating reactions.

Welding Slag - Slag deposits from welding are resistant to normal pick-ling acids and must be completely removed before the work enters the gal-vanizing process. Grinding or grit-/sand-blasting are strongly recommendedfor this purpose and are more effective than hand-chipping and wire-brush-ing. The removal of welding slag is the fabricators responsibility, unlessother arrangements have been made.

Rolling Defects in Steel - These defects may be broadly classified as dis-continuities in the steel that have been closed and elongated during rollingbut have not bonded. Examples are laminations, laps and folds, and non-metallic impurities rolled into metal surfaces. Defects of this type are some-times detected before or after pickling, but may not become apparent untilopened by the heat of the galvanizing bath. Minor flaws in the steel may beremoved by local grinding, but little reclamation is possible where the steelsurface is seriously defective.

Sand Embedded in Castings - This condition can result in localized bareareas. Since sand and other surface inclusions are not removed by conven-tional acid pickling, abrasive cleaning is generally required to provide aclean surface for galvanizing castings. This abrasive cleaning is typicallydone at the foundry before the parts are sent to the galvanizer.

Oxidized Steel - If the time between fluxing and galvanizing is pro-longed or the drying temperature is too high, the corrosion protection afford-ed the cleaned steel by a preflux may be lost. This is indicated by a rustyappearance on the ungalvanized article. The appearance of the galvanizedcoating is similar, in extreme cases, to that resulting from under-preparation.

Excess Aluminum - A condition sometimes referred to as black spotsmay occur if the aluminum content of a galvanizing bath on which a flux

Inadequate surfacepreparation

Welding slag

Improperly cleaned castings

10

blanket is used becomes too high. Minimal trouble should be experienced ifthe aluminum content of the bath is maintained below approximately 0.01percent, which is well above the range needed to brighten the coating.

Articles in Contact - The zinc in the galvanizing bath should have freeaccess to all parts of the surface. Articles entering and passing through thegalvanizing bath should not be in tight contact with each other.General Roughness

A rough coating is usually caused by excessive growth or unevenness ofthe alloy layers. This condition is attributable to the steels chemical com-position or its original surface condition. Since the irregularity of the alloylayers tends to increase with their thickness, heavy coatings are usuallyrougher than lighter ones. Where a heavy coating results, some degree ofroughness may be unavoidable. The importance attached to surface rough-ness varies with the nature of the product. For certain articles, such as tubeand pipe, which are sold largely on the basis of visual appeal, a smoothappearance may be essential. Also, where one surface is required to matewith another, such as pole line insulator caps, a rough coating may be detri-mental to the intended product function or assembly. Such cases are theexception, however. In most instances, the degree of roughness is not criti-cal. Provided it is within reason and adhesion is good, the material should beaccepted.Dross Protrusions

Dross is the zinc-iron alloy that settles to the bottom of the zinc bath.Dross inclusions in the coating resulting from agitation of the dross layer canproduce surface protrusions. Because dross has a corrosion rate similar tozincs, it has little effect on the normal life of the coating and its presence inthe form of finely dispersed pimples is not seriously objectionable.However, extensive dross inclusions are normally grounds for rejectionsince they tend to make the coating more susceptible to mechanical damageand may cause premature discoloration of the surface upon weathering.Lumpiness and Runs

The coatings surface uniformity is controlled primarily by the drainageof the zinc as the work leaves the galvanizing bath. A lumpy and unevencoating results when the rate of withdrawal is too fast or when the bath tem-perature is too low to allow molten zinc to drain back into the bath as theitem is removed. Excessive zinc may also occur because of delayed drainagefrom bolt holes, folds, seams and other pockets where zinc collects, and is adirect consequence of product design. The additional zinc, though wasteful,is clearly not detrimental except in those instances where a smooth finish isessential. A similar effect may result when articles are withdrawn in contactwith each other.Flux Inclusions

Flux inclusions occur when the wet galvanizing process is employed. Inthe wet process, a layer of zinc-ammonium chloride is floated on top of themolten zinc. The material to be galvanized passes through the flux immedi-ately prior to immersion in the zinc bath. The flux is carefully pushed to theside in order for the item to be removed. Flux inclusions may originate inseveral ways. Stale kettle flux, for example, tends to adhere to the steelinstead of separating cleanly from the surface as the work is dipped. Thismay occur even with active flux if residual grease, scale, or other surfacecontaminants, which resist the cleansing action of the flux blanket, are pre-sent. In both instances, the inclusions are often associated with bare spots inthe coating. Resulting black spots formed by the included flux particles are


Rough, bumpy coatings

Lumpiness and runs

Dross protrusions

Flux inclusions

distinguishable from dirt smuts, splash marks and other less harmful typesof surface contamination by their characteristic tendency to pick up mois-ture. Flux deposits picked up from the baths surface as the work is with-drawn do not warrant rejection if the underlying coating is sound and theflux is removed.Ash Inclusions

Zinc ash is the oxide film that develops on the surface of the galvaniz-ing bath. As with flux, ash may be burnt on the steel during dipping orpicked up from the top of the bath during withdrawal. Ash inclusions canoccur on work that is cumbersome and requires slow withdrawal from thebath. This ash has no adverse effect on service life. Zinc ash that is not detri-mental to the appearance of the finished product or that does not interferewith the products function is not cause for rejection. Gross oxide lumps dueto improper skimming of the exit surface of the bath can reduce the effectivethickness of the coating and are unacceptable.Matte Gray or Mottled Coating

This condition develops during cooling and is caused by lack of a freezinc layer on the coating surface. It usually appears as a localized dull patchor spider web-like area on an otherwise normal surface, although in extremecases it may extend over the entire surface of the steel. It is not cause forrejection unless specifically stated and agreed to by the galvanizer and thefabricator.

A matte gray coating is most frequently found on heavy sections thatcool slowly, with certain types of steel, such as those with relatively high sil-icon or phosphorus content, or severely cold-worked steel, all of which mayexhibit abnormally rapid alloy growth.

Where the condition is caused by the nature of the base steel, the galva-nizer has no control over its occurrence. Galvanizers generally do not haveprior knowledge of a steels chemical composition. A lower galvanizingtemperature and shorter immersion time followed (if the type of product issuitable) by rapid quenching in water to arrest the alloy growth may be suc-cessful in marginal cases. However, such measures are not always effectiveand matte areas in the coating may be unavoidable.

Due to the steels chemical nature, these coatings are often thicker thanthe bright galvanized coatings and provide service life in proportion to theincreased thickness. After exposure, these coatings may take on a light yel-low to brown dusty appearance as the alloy layers weather. The appearanceof this light residue colored by the iron content of the corrosion-resistant lay-ers should not be considered a sign of failure. Rust Stains

These are caused by seepage from joints and seams after galvanizing orby material being stored under or in contact with rusty steel. Rust stains ofthis type are superficial and should not be confused with failure of the under-lying coating. Rust stains caused by seepage from an assembly can indicatea need for a modification of the design. Surface rust stains are not cause forrejection of the galvanized product.Wet Storage Stain

Wet storage stain is the buildup of zinc oxide and zinc hydroxides on thegalvanized surface. As the name implies, wet storage stain occurs when thesteel is exposed to a humid or moist environment without access to freelycirculating air. Tightly stacked or nested galvanized items are particularly


Ash inclusions

Dark gray area

Typical coating

Mottled, spider web-likeappearance

vulnerable to wet storage stain, especially if they are stored asunopened bundles for more than a few weeks.

Although in extreme cases the protective value of the coating maybe impaired, attack is often superficial, despite the relative bulkinessof the zinc hydroxide.

Where the surface staining is light and smooth without growth ofthe zinc oxide layer, as judged by lightly rubbing fingertips across thesurface, the staining will gradually disappear and blend in with the sur-rounding zinc surface as a result of normal weathering in service. If theaffected area is not fully exposed in service or is subject to a humidenvironment, wet storage stain must be removed, even if it is superfi-cial, to allow formation of the basic zinc carbonate film, which nor-

mally contributes to the corrosion resistance of the galvanized coating.Medium to heavy buildup of white corrosion product must be removed;

otherwise, the essential protective film of basic zinc carbonates cannot formin affected area. Light deposits can be removed by brushing with a stiff bris-tle (not wire) brush. A coating thickness check should be performed on theaffected areas to ensure that sufficient zinc coating remains after the removalof the wet storage stain.

In advanced stages of wet storage stain, the typical white or gray corro-sion product may blacken. When this occurs, a significant amount of coat-ing has been lost to corrosion and the service life is decreased.

In extreme cases where heavy white deposits or red rust have formed asa result of prolonged storage under poor conditions, corrosion products mustbe removed and the damaged area repaired as detailed in ASTM A 780.Where the affected area is extensive, or when the wet storage stain wouldimpair the use of the article for its intended service, re-galvanizing may benecessary.

Unless present prior to shipment from the galvanizer, the developmentof wet storage stain is not cause for rejection. The customer must exerciseproper caution during transportation and storage to protect against wet stor-age stain. More information on wet storage stain can be found in the AGAPublication, Wet Storage Stain.

MISCELLANEOUS TESTSTesting for Chromate Finishes

In some cases, post-galvanizing chromate treatments are specified forthe prevention of wet storage stain. The presence of chromate film on thesurface of the galvanized coating is usually visible as a light yellow tint onthe surface.

ASTM B 201 describes a test method for determining the presence ofchromate coatings.Embrittlement Testing

Embrittlement of galvanized steel is very rare and usually is the result ofusing high-strength steel. The design of the product and selection of theproper steel for its suitability to be fabricated and galvanized without embrit-tlement are the responsibility of the designer and fabricator. Good commu-nication among the designer, fabricator and galvanizer can reduce the likeli-hood of encountering embrittlement.

AMERICAN GALVANIZERS ASSOCIATION12

Wet storage stain

As noted in the AGA publication The Design of Products to be Hot-DipGalvanized After Fabrication, the hot-dip galvanizing process produces nosignificant changes in the mechanical properties of the structural steel com-monly galvanized throughout the world.

In the rare case when embrittlement testing is specified, ASTM A 143and CSA G 164-M designate the appropriate test method to be used.

TOUCH-UP AND REPAIRWhen is Touch-up Necessary?

Occasionally during the hot-dip galvanizing process, bare spots or minorimperfections may occur that, if allowed to go un-repaired, will allow basemetal corrosion. Sometimes after leaving the galvanizers plant, the coatingis damaged during shipping or by welding during field erection. If the areaof renovation is less than or equal to 1" (25mm) in its narrowest dimensionand the total area is less than of 1% of the surface area to be coated on thearticle, or 36 square inches (22,5002 mm) per ton of piece weight, whichev-er is less, as allowed by ASTM A 123 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products, the area may berepaired. If the area to be repaired exceeds those limits, the material must bere-galvanized.Specifications

As cited above, ASTM A 123 specifies the maximum area that can berepaired, but ASTM A 780-00 Standard Practice for Repair of Damagedand Uncoated Areas of Hot-Dip Galvanized Coatings addresses most otherareas relevant to repair, including preparation, coating thickness, composi-tion of repair material and measurement.Surface Preparation

Regardless of what repair method is chosen, it is critical that whetherwire brushing, grinding or mild blasting is used to prepare the area, theundamaged surrounding galvanized area is disturbed as little as possible. Fornewly galvanized steel, this may mean light wire brushing of the bare areasand only high-pressure water washing of the surrounding galvanized area.For steel with long-term exposure, the bare or damaged areas may requiresand blasting and only a mild caustic solution washing of the surroundinggalvanized area.Touch-up and Repair Methods

There are three accepted repair methods: Spraying the bare spot or damaged area with metallic zinc (metallizing), Application of paint containing zinc dust, and Coating the affected area with zinc solder.

METALLIZING (ZINC SPRAYING)Definition

Metallizing is the melting of zinc powder or zinc wire in a flame or elec-tric arc and projecting the molten zinc droplets by air or gas onto the surfaceto be coated. The zinc used is nominally 99.5% pure or better; the perfor-mance of wire versus powder is the same. Zinc-aluminum alloys can also beused. The application equipment may limit the concentration of aluminum.


Wire brushing and painting

Washing, brushing and rinsing

Surface PreparationAccording to ASTM A 780, the surface to be reconditioned shall be blast

cleaned to SSPC-SP5/NACE No. 1 white metal and be free of oil, grease,weld flux residue, weld spatter and corrosion products. The blast cleaningmust extend into the surrounding undamaged galvanized coating.Application

Zinc spraying of the clean, dry surface by skilled workers should takeplace as soon as possible after preparation (within four hours) and prior todevelopment of visible oxides. Spraying in horizontal overlapping linesyields a more uniform thickness than the cross-hatch technique. The zinccoating can be sealed with a thin coating of low viscosity polyurethane,epoxy-phenolic, epoxy, or vinyl resin. For details of the applicationsequence and procedures, consult ANSI/AWS C2.18-93.

Zinc spraying can be done either in the galvanizers plant or on the jobsite, but the transport of blasting and metal spraying equipment to the jobsite may make it uneconomical compared to other touch-up and repair meth-ods. If high humidity conditions exist during spraying, adhesion may bedegraded.Performance Characteristics

Coating Thickness: The renovated area shall have a zinc coating thick-ness at least as thick as that specified in ASTM A 123/123M for the thick-ness grade for the appropriate material category. When blast cleaning is notavailable, wire brushing to expose bare steel is allowed. Thickness mea-surements should be taken with either a magnetic, electromagnetic or eddy-current gauge.

Corrosion Resistance: The usual criterion for determining the expectedservice life of zinc coatings is thickness: the thicker the coating, the longerthe service life. This is acceptable when comparing coatings produced by thesame process. However, because zinc sprays have a coating density less thanhot-dip galvanized coatings, 1.9 mils of zinc spray are needed to provide thesame amount of zinc as 1.7 mils of hot-dip galvanized coating. However,performance equivalency should not be inferred from this. Exposure condi-tions will determine true performance.

Coating Appearance: The surface of the sprayed zinc coating on therepair area should be free of lumps, coarse areas and loose particles. Touch-up and repair by metal spraying of a surface area surrounded by galvanizedsteel 18 months or older delivers an excellent color match. Some sprays withaluminum additives may also be a good match for newly galvanized, brightsurfaces. Uniformity of the coating is largely dependent on the skill of theworker.

Adhesion: Adhesion of the zinc spray to the base metal is by mechani-cal means and is dependent on the quality of surface preparation and clean-ing. The higher the surface profile, the better the mechanical bond. Adhesionvalues of 1000 psi are typical. The temperature of the zinc upon impact withthe base metal is not high enough to result in the alloy coating produced byhot-dipping.

Abrasion Resistance: Abrasion resistance of zinc spray coatings ismoderate compared to hot-dip galvanized surfaces.

Mechanical Properties: The relatively low temperature of applicationhas no adverse effect on the steel properties. Metallizing does improve some



properties such as surface frictional coefficients and corrosion fatigue resis-tance.

High Temperature: Sprayed zinc coatings are suitable for constantexposures up to approximately 390 F (200 C) and short-term exposure athigher temperatures.

ZINC-RICH PAINT APPLICATIONDefinition

Touch-up using zinc-rich paint is the spray or brush application of a zincand usually organic binder pre-mix to a clean, dry steel surface. Zinc dustpaints must contain between 65% and 69% zinc by weight or greater than92% metallic zinc in dry film. Paints containing zinc dust are classified asorganic or inorganic, depending on the binder. Inorganic binders are partic-ularly suitable for paints applied in touch-up applications around and overundamaged hot-dip galvanized areas.Surface Preparation

According to ASTM A 780, the surface to be reconditioned shall be blastcleaned to SSPC-SP10/NACE No. 2 near white metal for immersion appli-cations and SSPC-SP11 bare metal for less aggressive field conditions.When blasting or power tool cleaning is not practical, hand tools may beused to clean areas to be reconditioned. In all cases, the surface must be dry,free of oil, grease, weld flux, preexisting paint and corrosion byproducts.The blast cleaning must extend into the surrounding undamaged galvanizedcoating.Application

Paints containing zinc dust may be applied by brush or spray on a clean,dry surface by skilled workers. Painting should take place as soon as possi-ble after preparation and prior to development of visible oxides. Spraying orbrushing should be in a single application of multiple passes according to thepaint manufacturers written instructions. Proper curing of the repaired areamust occur before the article is returned to service. Zinc painting can bedone in the galvanizers plant or on the job site and is the easiest repairmethod to apply because limited equipment is required. If high humidityand/or low temperature conditions exist during zinc painting, adhesion maybe adversely affected.Performance Characteristics

Coating Thickness: The renovated area shall have a zinc coating thick-ness of 150% of that specified in ASTM A 123 for the thickness grade forthe appropriate material category, but not more than 4 mils. Thickness mea-surements should be taken with either a magnetic, electromagnetic or eddy-current gauge.

Corrosion Resistance: Zinc-rich paints contain greater than 65% zinc inthe dry film condition. High concentrations of zinc may provide cathodicprotection in addition to barrier protection. Exposure conditions will deter-mine true coating corrosion protection performance. Inorganic zinc-richpaints are more effective than organic in terms of delivering corrosion pro-tection.

Coating Appearance: The surface of the painted coating on the repairarea should be free of lumps, coarse areas and loose particles. Touch-up andrepair materials are formulated to deliver an excellent color match for bothnewly galvanized, bright finish coatings and for matte gray, aged galvanized

Zinc-rich paint application


coatings. Inorganic zinc-rich coatings do not shrink after drying/curing asorganic coatings do. This is a major advantage when the coating is applied tocorners, edges and rough surfaces. However, when inorganic coatings areapplied too thickly, mud-cracking may occur. Uniformity of the coating islargely dependent on the skill of the worker.

Adhesion: Bond strength for paints containing zinc dust are on the orderof a few hundred psi. Adhesion of the paint is largely a function of the clean-liness of the surface being repaired. There is no alloying when applying thepaint; the bond is simply mechanical.

Abrasion Resistance: Abrasion resistance of zinc-containing paint coat-ings is minimal compared to hot-dip galvanized surfaces. The limited ductil-ity of paints containing zinc dust gives them poor impact resistance.

Mechanical Properties: Some paints containing zinc dust provideenough friction resistance to allow them to be used on faying surfaces, deliv-ering a coefficient of friction equal to or greater than the 0.5 values obtainedby sandblasting steel surfaces.

High Temperature: Inorganic coatings provide effective service at tem-peratures up to 700 F (370 C). Organic zinc-rich paints do not have the tem-perature resistance of inorganic zinc-rich paints and are limited to servicetemperatures of 200 to 300 F (90 to 150 C).

SOLDERING WITH ZINC-BASED ALLOYSDefinition

Soldering with zinc-based alloys is defined as applying zinc alloy in stickor powder form to the area to be repaired that has been preheated to approx-imately 600 F (315 C). Common solders used for repair include zinc-tin-lead,zinc-cadmium and zinc-tin-copper alloys.Surface Preparation

According to ASTM A 780, the surface to be reconditioned shall be wire-brushed, lightly ground or mild blast cleaned. All weld flux and spatter mustbe removed by mechanical methods if wire brushing or light blasting areinadequate. The cleaned area should be preheated to 600 F (315 C) and at thesame time wire brushed. Care should be exercised to not burn the surround-ing galvanized coating.Application

Solders are the most difficult of the three repair methods to apply. Cautionmust be taken while heating the bare spot to prevent oxidizing the exposedsteel or damaging the surrounding galvanized coating. Because solders aremolten when applied, resultant coatings are inherently thin. When the repairhas been completed, remove the flux residue by rinsing with water or wipingwith a damp cloth. Solders are typically not economically suited for touch-upof large areas because of the time involved in the process and because heat-ing large areas to the same temperature is very difficult.Performance Characteristics

Coating Thickness: The renovated area shall have a zinc coating thick-ness at least as much as that specified in ASTM A 123 for the thickness gradefor the appropriate material category, but not more than 4 mils. Thicknessmeasurements should be taken with either a magnetic, electromagnetic oreddy-current gauge. Operator skill is particularly important to ensure consis-tent coating thickness across the repair area.

Corrosion Resistance: Because of the relatively thin film of zinc thatcan be applied, long-term performance tests of zinc-based solders indicatethey do not perform as well as metallizing or painting with zinc. Somecathodic and barrier protection is provided by solders.

Coating Appearance: If solder material chemistry is chosen to matchthe galvanized coating, solders can deliver a very good color match.

Adhesion: Because there is heating of the surface to be repaired to 600F (315 C) there may be some alloy layer development between the basemetal and the zinc. Thus, bond strength for solders is very good.

Abrasion Resistance: Based on tests performed according to ASTM D968 Determination of Abrasion Resistance by the Falling Sand Method,abrasion resistance of solders is minimal compared to hot-dip galvanizedsurfaces and even metallized surfaces.

Mechanical Properties: Solder materials have little or no effect on themechanical properties of the underlying steel. Solders in general have asmooth surface and very low coefficients of friction so they should not beused in the area of faying surfaces.

High Temperature: Solders can withstand constant temperature expo-sure of approximately 550 F (285 C); the surrounding galvanized steel coat-ing performs well only to constant temperature exposure of about 390 F (200C).Repair Method SelectionConsiderations

Performance: The touch-up and repair method chosenshould be made after consider-ing the specific requirementsof the application and the per-formance characteristics ofeach of the three touch-upmethods. Corrosion protectionshould always be the primaryconsideration, but certain usesand conditions may warrantselection on the basis of one ofthe other performance charac-teristics.

Economics: The locationof the article to be repaired, thesize of the area and the skilllevel of the repair labor are thethree primary determinants foreconomic consideration.


COMPARATIVE PERFORMANCE RATINGSOF REPAIR METHODS

Metallizing Zinc Dust Zinc-basedPaints Solder

Corrosion protectionBarrier Very Good Good FairCathodic Excellent Poor Fair

Coating appearance Very Good Very Good GoodAdhesion Very Good Poor Very GoodAbrasion resistance Fair Poor PoorMechanical properties Good Good PoorHigh temperature Good Good Very Good

ECONOMIC EVALUATION OF REPAIR METHODSMetallizing Zinc Dust Zinc-based

Paints SolderEquipment/materials $$$ $ $$$In-plant $ $ $Field $$$ $ $$$Preparation $$ $$ $Small touch-up area $$ $ $$Large touch-up area $$ $ $$$Skill level of labor $$$ $$ $$$

$ low investment$$ medium investment$$$ high investment


ASTM A 90 Test Method for Weight of Coating on Zinc-Coated(Galvanized) Iron or Steel Articles

ASTM A 123 Zinc (Hot-Dip Galvanized) Coatings of Iron andSteel Products

ASTM A 143 Practice for Safeguarding Against Embrittlement ofHot Dip Galvanized Structural Steel Products andProcedure for Detecting Embrittlement

ASTM A 153 Zinc Coating (Hot Dip) on Iron and Steel HardwareASTM A 325 High-Strength Bolts for Structural Steel Joints,

Including Suitable Nuts and Plain Hardened WashersASTM A 384 Recommended Practice for Safeguarding Against

Warpage and Distortion During Hot Dip Galvanizingof Steel Assemblies

ASTM A 385 Recommended Practice for Providing High QualityZinc Coatings (Hot Dip) on Assembled Products

ASTM A 394 Steel Transmission Tower BoltsASTM A 767 Zinc Coated (Galvanized) Steel Bars for Concrete

ReinforcementASTM A 780 Practice for Repair of Damaged Hot-Dip Galvanized

CoatingsASTM B 6 Zinc Metal (Slab Zinc)ASTM C 633 Standard Test Method for Adhesion or Cohesive

Strength of Flame-Sprayed CoatingsASTM D 968 Determination of Abrasion Resistance of Paint by

Falling Sand MethodASTM E 376 Recommended Practice of Measuring Coating

Thickness by Magnetic-Field or Eddy-Current(Electromagnetic) Test Methods

G 40.8* Structural Steel With Improved Resistance to BrittleFracture

G 40.12* General Purpose Structural SteelGeneral Requirements for Rolled or WeldedStructural Quality Steel

G 164 Galvanizing of Irregularly Shaped Articles*Superseded by G 40.20/G 40.21

The Design of Products to be Hot-Dip Galvanized After FabricationAmerican Galvanizers Association, Aurora, Colo., 2000

Recommended Details for Galvanized StructuresAmerican Galvanizers Association, Aurora, Colo., 1984

Welding Galvanized SteelAmerican Galvanizers Association, Aurora, Colo., 1992

Wet Storage StainAmerican Galvanizers Association, Aurora, Colo., 1997

A Comparison of Touch-up Materials for Galvanized ProductsA Cominco study

RelatedRelatedAmericanAmericanSociety ofSociety ofTTesting andesting andMaterialsMaterialsSpecificationsSpecifications

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FFurtherurtherReading andReading andRelatedRelatedMaterialsMaterials


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