ITEM 733 - ZINC (HOT-DIP GALVANIZED) COATINGS ON IRON AND STEEL PRODUCTS
733.1
Description
733.1.1
Scope
This
specification covers the requirements for zinc coating (galvanizing) by the
hot-dip process on iron and steel products made from rolled pressed and forged
shapes, castings, plates, bars and strips.
This
specification covers both fabricated and unfabricated products, for example,
assembled steel products, structural steel fabrications, large tubes already
bent or welded before galvanizing, and wire work fabricated from uncoated steel
wire. It also covers steel forgings and iron castings incorporated into pieces
fabricated before galvanizing or which are too large to be centrifuged (or
otherwise handled to remove excess galvanizing bath metal).
733.2
Materials Requirements
733.2.1
Steel or Iron
The
specification, grade or designation, and type and degree of surface
contamination of the iron or steel in articles to be galvanized shall be
supplied by the purchaser to the hot-dip galvanizer prior to galvanizing.
The
presence in steels and weld metal, in certain percentages, of some elements
such as silicon, carbon and phosphorus tends to accelerate the growth of the
zinc-iron alloy layer so that the coating may have a matte finish with a little
or no outer zinc layer.
733.2.2
Fabrication
The
design and fabrication of the product to be galvanized shall be in accordance
to the plans and specifications. ASTM A 143, A 384 and A 385 provide guidance
for steel fabrication for optimum hot dip galvanizing and shall be complied
with in both design and fabrication.
733.2.3
Castings
The
composition and heat treatment of iron and steel castings shall conform to
specifications designated by the purchaser. Some types of castings have been
known to show potential problems being embrittled during the normal
thermal'(cycle of hot-dip galvanizing. The requirements for malleable iron
castings to be galvanized are stipulated in ASTM A 4 7.
733.2.4
Zinc
The zinc used in the galvanizing bath shall
conform to ASTM B 6. If a zinc alloy is used as the primary feed to the
galvanizing bath, then the base material used to make that alloy shall conform
to ASTM B 6.
733.2.5
Bath Composition
The molten metal in the working volume of the
galvanizing bath shall contain not less than an average value of 98.0% zinc by
weight.
733.3
Coating Properties
Table 1 - Minimum Average Coating Thickness Grade by
Material Category
|
Material Category
|
All Specimens Tested
Steel Thickness Range (Measured), mm
|
||||
|
<1.6
|
1.6 to <3.2
|
3.2 to 4.8
|
>4.8 to <6.4
|
≥6.4
|
|
|
Structural
Shapes & Plate Strip and Bar
Pipe
and Tubing
Wire
|
45
45
45
35
|
65
65
45
50
|
75
75
75
60
|
85
85
75
65
|
100
100
75
80
|
733.3.1
Coating Thickness
The
average thickness of coating for all specimens tested shall conform to the
requirements of Table 1 for the categories and thicknesses of the material
being galvanized. Minimum average thickness of coating for any individual
specimen is one coating grade less than that required in Table 1. Where
products consisting of various material thicknesses or categories are
galvanized, the coating thickness grades for each thickness range and material
category of material shall be as shown in Table 1. The specification of coating
thickness heavier than those required by Table 1 shall be subject to mutual
agreement between the galvanizer and Engineer.
For
articles whose surface area is greater than 100,000 mm2 (multispecimen
articles), each test article in the sample must meet the appropriate minimum
average coating thickness grade requirements of Table 1. Each specimen coating
thickness grade comprising that overall average for each test article shall
average not less than one coating grade below that required in Table 1.
For
articles whose surface area is equal to or less than 100,000 mm2
(single-specimen articles), the average of all test articles in the sample must
meet the appropriate minimum average coating thickness grade requirements of
Table 1. For each test article, its specimen coating thickness shall not be
less than one coating grade below that required in Table 1.
No
individual measurement or cluster of measurements at the same general location
on a test specimen shall be cause for rejection under this specification
provided that when those measurements are averaged with the other dispersed
measurements to determine the specimen coating thickness grade for that
specimen, the requirements of the above specifications as appropriate are met.
The
coating thickness grades in Table 1 represent the minimum value obtainable·
with a high level of confidence for the ranges typically found in each material
category. While most coating thicknesses will be in excess of those values,
some materials in each category may be less reactive (for example, because of
chemistry or surface condition) than other materials of the steel category
spectrum. Therefore, some articles may have a coating grade at or close to the
minimum requirements shown in Table 1. In such cases, the precision and
accuracy of the coating thickness measuring technique should be taken into
consideration when rejecting such articles for coating thickness below that is
required by this specification.
733.3.2
Finish
The
coating shall be continuous (except as provided below), and as reasonably
smooth and uniform in thickness as the weight, size and shape of the item.
Except for local excess coating thickness which would interfere with the use of
the product or make it dangerous to handle (edge tears or spikes), rejection
for non- uniform coating shall be made only for plainly visible excess coating
not related to design factors such as holes, joints, or special drainage
problems. Since surface smoothness is a relative term, minor roughness that
does not interfere with the intended use of the product, or roughness that is
related to the as-received (un-galvanized) surface condition, steel chemistry
or steel reactivity to zinc shall not be grounds for rejection.
Surfaces
that remain uncoated after galvanizing may be renovated in accordance with the
methods in ASTM A 780 provided that the following conditions are met:
1. Each area subject to renovation shall
be 25 mm or less in its narrowest dimension.
2. The total area subject to renovation on
each article shall be no more than % of 1 % of the accessible surface area to
be coated on that article, or 22,500 mm2 per ton of piece· weight, whichever is
less. Inaccessible surface areas are those which cannot be reached for
appropriate surface preparation and application of repair materials as
described in ASTM A 780.
3. The thickness o{ renovation shall be
that is required by the thickness grade for the appropriate material category
and thickness range in Table 1 in accordance with the coating thickness
requirements, except that for renovation using zinc paints, the thickness of
renovation shall be 50% higher than that required by Table 1, but not greater
than 0.0254 mm.
4.
When areas requiring renovation
exceed the criteria previously provided, or are inaccessible for repair, the
coating shall be rejected.
733.3.3
Threaded Components in Assemblies
The
zinc coating on external threads shall not be subjected to a cutting, rolling,
or finishing tool operation, unless specifically authorized by the purchaser.
Internal threads may be tapped or retapped after galvanizing. Coatings shall
conform to the · requirements of ASTM A 153/A 153 M.
733.3.4
Appearance
Upon
shipment from the galvanizing facility, galvanized articles shall be free from
uncoated areas, blisters, flux deposits, and gross dross inclusions. Lumps,
projections, globules or heavy deposits of zinc which will interfere with the
intended use of the material will not be permitted. Plain holes of 12.5 mm
diameter or more shall be clean and reasonably free from excess zinc. Marks in
the zinc coating caused by tongs or other items used in handling the article
during the galvanizing operation shall not be cause for rejection unless such
marks have exposed the base metal, and the bare metal areas exceed the criteria
provided in number 1 and 2 of Subsection 733.3.2, Finish.
Whenever
dross is present in a form other than finely dispersed pimples in the coating
and is present in such amount as to be susceptible to mechanical damage, it
will be considered as "gross".
733.3.5
Adherence
The
zinc coating shall withstand handling consistent with the nature and thickness
of the coating and the normal use of the article, without peeling or flaking.
Although some material may be formed after galvanizing, in general the zinc
coating on the articles covered by this specification is too heavy to permit
severe bending without damaging the coating.
733.4
Sampling
A
lot is a unit of production or shipment from which a sample may be taken for
testing. Unless otherwise agreed upon between the galvanizer and the purchaser,
or established within this specification, the lot shall be as follows:
1.
For testing at a galvanizer's
facility, a lot is one or more articles of the same type and size comprising a
single order or a single delivery load, whichever is smaller:; or any number of
articles identified as a lot by the galvanizer, when these have been galvanized
within a single production shift and in the same bath.
2. For test by the purchaser after
delivery, the lot consists of the single order or the single delivery load,
whichever is smaller, unless the lot identity, established in accordance with
the above, is maintained and clearly indicated in the shipment by the
galvanizer.
The method of selection and number of test
specimens shall be agreed upon between the galvanizer and the purchaser. Otherwise,
the test specimens shall be selected at random from each lot. In this case, the
minimum number of specimens from each lot shall be as follows:
Number of Pieces in Lot Number of Specimens
3 or
less All
4 to
500 3
501
to 1200 5
1201
to 3200 8
3201
to 19000 13
10001
and, over 20
A
test specimen which fails to conform to any requirement of this specification
shall not be used to determine the conformance to other requirements.
733.5
Test Requirements
733.5.1
Thickness of Coating Test
733.5.1.1
Magnetic Thickness Measurements
The
thickness of the coating shall be determined by magnetic thickness gage
measurements in accordance with ASTM PE 376. For each specimen, five or more measurements
shall be made at points widely dispersed throughout the volume occupied by the
specimen so as to represent as much as practical, the entire surface area of
the test specimen. The average of the five or more measurements thus made for
each specimen is the specimen coating thickness.
For
articles whose surface area is greater than 100,000 mm2 the average
of the three specimen coating thickness grades comprising each test article is
the average coating thickness for that test article. A specimen must be
evaluated for each steel category and material thickness within the
requirements for each specimen of the test article.
For
articles whose surface area is equal to or less than 100,000 mm2 the
average of all specimen coating thickness grades is the, average coating
thickness for the sample.
The
use of magnetic measurement method is appropriate for larger articles, and may
be appropriate for smaller articles when such is practical using ASTM E 376.
733.5.1.2
Stripping Method
The
average weight of coating may be determined by stripping a test article, a
specimen removed from a test article, or group of test articles in the case of
very small items such as nails, etc., in accordance with ASTM A 90/A 90M. The
weight of coating per unit area thus determined is converted to equivalent
coating thickness values in accordance with Table 2, Coating Thickness Grade
(rounding up or down as appropriate). The thickness of coating thus obtained is
the test article coating thickness, or in the case of a specimen removed from a
test article, is the specimen average coating thickness.
Table 2 -Coating Thickness GradeA
|
Coating Grade
|
µm
|
g/m2
|
|
35
45
50
55
60
65
75
80
85
100
|
35
45
50
55
60
65
75
80
85
100
|
245
320
355
390
425
460
530
565
600
705
|
A Conversions
in Table 2 are based on the metric thickness value equivalents from the next
earlier version of this specification, using conversion factors consistent with
Table X2.1 in A 653/A 653M, rounded to the nearest 5 µm. The conversion factor
used is: g/m2 = µm x 7.067.
733.5.1.3
Weighing Before or After Galvanizing
The
average weight of coating may be determined by weighing articles before and
after galvanizing, subtracting the first weight from the second and dividing
the result by the surface area. The first weight shall be determined after
pickling and drying, and the second after cooling to ambient temperature. The
weight of coating per unit area thus determined is converted to equivalent
coating thickness values according to Table 2 (rounding up or down as
appropriate). The thickness of coating thus obtained is the test article
coating thickness.
733.5.1.4
Microscopy
The
thickness of coating may be determined by cross-sectional and optical
measurement in accordance with ASTM B 487. The thickness thus determined is a
point value. No less than five such measurements shall be made at locations on
the test article which are as widely dispersed as practical, so as to be
representative of the whole surface of the test article. The average of no less
than five such measurements is the specimen coating thickness.
733.5.2
Adhesion
Determine
adhesion of the zinc coating to the surface of the base metal by cutting or
prying with the point of a stout knife, applied with considerable pressure in a
manner tending to remove a portion of the coating. The adhesion shall be
considered inadequate if the coating flakes off in the form of, a layer of the
coating so as to expose the base metal in advance of the knife point. Do not
use testing carried out at edges or corners (points of lowest coating adhesion)
to determine adhesion of the coating. Likewise, do not use removal of small
particles of the coating by paring or whittling to determine failure.
733.5.3
Embrittlement
Test
for embrittlement may be made in accordance with ASTM A 143.
The
galvanized article should withstand a degree of bending substantially the same
as the ungalvanized article. Flaking or spalling of the galvanized coating is
not to be construed as an embrittlement failure.
733.6
Inspection, Rejection and Retest
The
material shall be inspected at the galvanizer's plant prior to shipment.
However, by' agreement the purchaser may make the tests which govern the
acceptance or rejection of the materials in his own laboratory or elsewhere.
When
inspection of materials to determine conformity with the visual requirements of
Subsection 733.3.2, Finish warrants rejection of a lot, the galvanizer may sort
the lot and submit it once again for acceptance after he has removed any
nonconforming articles and replace them with conforming articles.
Materials
that have been rejected for reasons other than embrittlement may be stripped
and regalvanized, and again submitted for inspection and test at which time
they shall conform to the requirements of this inspection.
733.7
Transport and Storage
Galvanized
components shall, wherever possible, be transported and stored under dry,
well-ventilated conditions to prevent the formation of wet storage staining.
Either
zinc phosphate or chromate passivation treatment after galvanizing may be used
to minimize the wet storage staining which may occur on articles unable to be
stored in dry, well-ventilated conditions.
Provided
the coating thickness complies with the requirements of Subsection 733.3.1,
Coating Thickness, no further remedial action is required to the stained areas.
733.8
Measurement and Payment
Zinc
(Hot-dip galvanized) coating shall not be measured and paid for separately, but
the cost thereof shall be considered as included in the contract unit price of
the Items where called for.
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