Inspeccion de Galvanizado

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AMERICAN GALVANIZERS ASSOCIATION The Inspection of Products to be Hot-Dip Galvanized After Fabrication Including a New Section on Touch-Up and Repair
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Transcript of Inspeccion de Galvanizado

The Inspection of Products to be Hot-Dip Galvanized After FabricationIncluding a New Section on Touch-Up and Repair

AMERICAN G ALVANIZERS ASSOCIATION

THE INSPECTION OF PRODUCTS H OT-DIP GALVANIZED AFTER FABRICATIONTABLE OF CONTENTSPurpose of Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Sampling for Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Coating Microstructure and Causes of Thickness and Uniformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 Thickness Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Magnetic Thickness Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Stripping and Weighing Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Weighing Before and After Galvanizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Thickness Testing Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Magnetic Balance Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Pull-off Magnetic Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Electronic Thickness Gauges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Coating Adherence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Testing for Adherence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Appearance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Pre-galvanizing Consultation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 Visual Inspection Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 Bare Spots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 General Roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Dross Protrusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Lumpiness and Runs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Flux Inclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 Ash Inclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Matte Gray or Mottled Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Rust Stains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Wet Storage Stain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Miscellaneous Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Testing for Chromate Finishes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Embrittlement Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Touch-up and Repair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 When is Touch-up Necessary? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Touch-up and Repair Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Metallizing (Zinc Spraying) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 Performance Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

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

AMERICAN G ALVANIZERS ASSOCIATION

TABLE OF CONTENTS CONT.

Zinc-rich Paint Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 Performance Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 Soldering with Zinc-based Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Performance Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Repair Method Selection Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17 Related Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

AMERICAN GALVANIZERS ASSOCIATION

PURPOSE OF INSPECTIONHot-dip galvanizing after fabrication is one of the most widely used methods of corrosion protection. The final step in the hot-dip galvanizing process is inspection to ensure compliance with specifications. Interpretation of inspection results must be made with a clear understanding of the causes of various coating conditions and their affects on the ultimate objective of providing corrosion protection. First and foremost, the purpose of hot-dip galvanizing is to protect steel from corrosion. The length of time this protection can reasonably be expected to last before minimal maintenance is required is called its service life. Galvanizings service life is directly related to the thickness of the protective zinc coating: the thicker the coating the longer the service life (see Figure 1). Thus, coating thickness is the single most important inspection check to determine the quality of a galvanized coating. Coating thickness, however, is only one item of inspection. Coating uniformity, adherence and appearance also are evaluated. Additionally, embrittlement and other defects arising from fabrication and design are inspection concerns. While minimum standards must be satisfied in all these areas, their relative importance varies according to the end-use of the finished product. For example, the end-use requirement for galvanized structural steel in an isolated area differs from that for a thin-gauge product used in a decorative application. Understanding the individual requirements of the product and the capability of the hot-dip galvanizing process is essential for proper inspection. Inspection of the galvanized product is most effectively and efficiently conducted at the galvanizers plant. Here, questions and concerns can be raised and dealt with quickly and efficiently - speeding up the inspection process and resulting in a time savings that is an asset to the overall project.After items are removed from the galvanizing bath, the inspection process begins

SAMPLING F OR TESTSTo properly evaluate hot-dip galvanized coatings, it is essential that selected specimens be representative of the inspection lot. Because inspection lot sizes can be very small, statistical sampling plans, such as covered in American Society of Testing & Materials (ASTM) specification B 602 and Canadian Standards Association (CSA) specification Z 90, may not be applicable. However, such a statistical sampling plan is recommended for larger lots. The inspection lot is a collection of galvanized articles 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.

Figure 1

AMERICAN GALVANIZERS ASSOCIATION

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ASTM A 123, section 7, provides guidelines for the number of specimens that need to be tested per lot. For small objects such as nuts, bolts and washers, an entire article should be the test specimen. For large articles such as plates, bars and angle sections, tests should be conducted on the galvanized article according to the procedure described in ASTM A 123, section 6.1. The measurement of coating thickness should be taken at widely dispersed points to represent a true sampling of the whole part. Extremely large parts may need to be tested in sections in order to represent the entire product.Figure 2. Typical zinc/iron alloy layers

COATING MICROSTRUCTURE AND AND U NIFORMITYEta(100% Zn)

THE

CAUSES

OF

THICKNESS

Zeta

The thickness of the galvanized coating is the primary factor in determining its service life. The thicker the coating, the longer the corrosion protection lasts. The factors affecting coating thickness are a combination of several variables, some of which the galvanizer can control and some of which are beyond the galvanizers control. The chemical composition of the steel plays the biggest role in determining the thickness of a galvanized coating.

(94% Zn 6% Fe)

Delta

(90% Zn 10% Fe)

Gamma Steel

(75% Zn 25% Fe)

Figure 3. Irregular zinc/iron alloy layers

During the galvanizing process, a complex metallurgical reaction takes place creating a series of zinc-iron alloy layers. These layers contain varying amounts of zinc and iron, depending on the proximity to the base steel. The layers closest to the base steel contain more iron and less zinc than the layers farther away. Figure 2 depicts the zinc-iron alloy Free Zinc Layer layers as seen in a typical after-fabrication, hot-dip galvanized coating. Normally, the alloy-layer growth tapers off as the steel reaches the bath temperature. As the item is removed from the bath, a free zinc layer forms, giving the Zinc-Iron Alloy galvanized coating its familiar shiny, silver appearance.Layers

Figure 4. Coating weights on reactive steels

However, certain steel compositions tend to accelerate the growth of the zinc-iron alloy layers so that the galvanized coating has a matte finish with little or no pure zinc outer layer (see Figure 3). Steel containing carbon in excess Steel of 0.25 percent, phosphorus in excess of 0.04 percent, or manganese in excess of about 1.35 percent have been shown to create these heavy coatings. This coating also tends to be thicker than traditional bright, shiny galvanized coatings. The galvanizer has no control over this reactive silicon-killed steel condition, which is illustrated in Figure 4. These thicker coatings frequently have a dark gray, matte appearance due to the lack of a free zinc layer capping the alloy layers. A silicon level greater than 0.30 percent is particularly influential in producing heavier zinc-iron alloy layer coatings. The surface condition of the steel also influences the thickness and smoothness of the galvanized coating. Non-reactive steels that have been abrasively cleaned prior to galvanizing can have coating thicknesses 50 to 100 percent greater than steels only chemically cleaned. In reactive steels, the opposite effect can be seen with steels that have been abrasively cleaned. The coating thickness on these steels is generally AMERICAN G ALVANIZERS ASSOCIATION

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lower than expected. The appearance, however, remains a matte gray with occasional roughness. The mass, the shape and the amount of cold working of the piece also affect coating thickness and uniformity. When a fabricated article has both heavy and light sections, differences in coating thickness between the sections may result. Since immersion time varies according to the relationship of the surface area of an item to its weight, the galvanizer has little control over this situation. Variables the galvanizer can control are bath temperature and withdrawal rate. Because formation of the zinc-iron alloy layers is a diffusion process, higher bath temperatures generally produce heavier alloy layers. Like many diffusion processes, the reaction proceeds rapidly at first and slows down as the alloy layers become thicker. The thickness of the outer zinc layer largely depends on the rate of withdrawal from the zinc bath and the drain-off of excess zinc. A faster rate of withdrawal causes an article to carry out more zinc. This results in a heavier coating. When hot-dip galvanizing fabricated articles, local differences in the drain-off, because of the shape of the article and the angle at which different surfaces leave the bath, will generally result in some variation in coating thickness. Specifications for hot-dip galvanizing recognize that variations in coating thickness are inherent in the process. The minimum thickness of the zinc coating is always specified as an average thickness of specimens tested and a minimum weight for any individual specimen.

Appearance of reactive and non-reactive steels after galvanizing

When measurements are made to determine the thickness distribution of a large galvanized article, a sufficient number of readings, not less than five and preferably 10, should be taken at each end and in the middle of the article being examined (see Figure 5). The measurements in each area should be taken at least four inches from the edge to avoid end effects and as widely dispersed as possible. Usually, the end of an article that leaves the bath last will carry a thicker coat- Figure 5. Measurements should be taken at each end ing. This is particularly so towards the edge, where and in the middle of the piece at the time of drainage the last few drops of the zinc tend to collect as a result of surface tension. The minimum coating requirements specified by the ASTM for different classes of work are summarized in Table 1 for ASTM A 123, Table 2 for ASTM A 153 and Table 3 for ASTM A 767. The thickness of a zinc coating is reported in thousandths of an inch. For ease of conversion to thickness in inches and mils, one ounce of zinc per square foot is equivalent to 0.0018 inches or 1.8 mils. A conversion chart for common coating weights in both English and metric units is provided in Table 5. The CSA minimum coating requirements for different classes of work are 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 most likely be dictated by the size, shape and number of pieces to be tested. Some AMERICAN GALVANIZERS ASSOCIATION3

Table 1

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