I/EC A
CORROSION
W O R K B O O K
F E A T U R E
by J. W. Sicard, Ce/anese Corp. of America
All structural members in Celanese's new low pressure polyethylene plant are galvanized instead of painted structural steel
Galvanized Structural Steel in Gulf Coast Construction Celanese's experience at Bishop, Tex., has led to the use of galvanized, rather than painted, steel in its new, low-pressure polyethylene plant
W,' H E N Celanese Corp. of America
built its multimillion dollar Fortiflex plant for production of low-pressure polyethylene in 1956, galvanized structural steel was used in preference to painted steel in all exposed out door areas—including pipe supports, outdoor structures a n d platforms, ladders, and stairways. T h e plant, situated on a 200-acre site in the industrial area near the Houston Ship Channel, is exposed to moist winds from Galveston Bay, a salt water body. In addition to the troublesome Gulf Coast atmosphere, there was the d a n g e r of exposure of painted surfaces to hot process sol vent. After consideration of the low initial cost of galvanizing, with a n expected life of 10 to 15 years, Celanese elected to install galvanized steel structures. T o d a y , after 1 year of operation, the galvanized structures present a n attractive a p pearance, and there is n o sign of corrosion. Pleasing a p p e a r a n c e and
protection from atmospheric cor rosion were attained at a low cost. G a l v a n i z i n g vs. Painting
Celanese based its use of galvan ized steel on experience gained in the Gulf area at its petrochemical plant located near Bishop, Tex. Experi ence there has shown paint mainte nance to be costly, requiring new finish coats every 4 years. O n the other hand, galvanized steel cat
walks at Bishop are still in good con dition after 10 years of service. Actual initial costs for galvanizing in the Houston Plastics Plant turned out to be only $0.20 per square foot, as compared to a n estimated $0.20 to $0.30 per square foot for applica tion of paint. T h e initial cost of galvanizing is compared with paint ing in the table below. T h e estimated paint cost is con sidered very conservative; Burns and Bradley (2) gave cost d a t a for a
Galvanizing vs. Painting Structural Steel (Initial cost per square foot) Painting'' Galvanizing" Case A Case Β Wire brush Sandblast $0.15 and chip $0.05 Primer 0.05 Primer 0.05 2 finish coats 0.10 2 finish coats 0.10 Total $0.20 $0.30 $0.20 " Average cost on 200 tons of various-sized pipe supports, based on cost of $50 per ton for 2-ounce coat. b Primer applied as shopeoat; finish coats applied in field.
VOL. 49, NO. 12
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DECEMBER 1957
69 A
I/EC
CORROSION
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A Workbook Feature
three-coat application showing $0.32 per square foot. However, as it is assumed that the .primer will be applied by the fabricator as a shopcoat, the lower cost is reasonable. Transportation
As rapid construction was a prime factor in Celanese plant planning, time requirements to galvanize could not be allowed to retard construc tion schedule. Actually, negligible time was required for galvanizing, a n d steel was delivered on time to t h e construction site. Galvanizing costs are high if overlength beams are used. I n the hot-dip trade, beams longer t h a n the zinc bath must be doubledipped and any uncoated portion in the center metallized. Typical bath dimensions are 30 feet in length, 4 feet in width, a n d 4 feet in depth. Fortunately, overlength beams were used only in r o a d spans at the Houston Fortiflex Plant, which rep resented about 5 % of total structural steel tonnage. Steel was delivered in cars directly from the fabricator to a local galvanizer, thereby eliminating freight on the weight of zinc coating. T h e galvanized steel was then rerouted by rail to the construction site. Railroad switching charges at the galvanizing facility were slight, a m o u n t i n g to about $30 per car. Slag R e m o v a l
Flux a n d slag must be removed be fore galvanizing to attain a perfect zinc coat. Usually the steel fabri cator is required to remove the bulk of the slag, and any remaining will be removed at the galvanizing facility. However, it is advisable to get com plete removal by the fabricator, as charges for this service by the galvanizer m a y be appreciable. Welding
Welding to galvanized steel is avoided wherever possible by using field bolted construction with gal vanized bolts, nuts, h a r d w a r e , etc. Welding galvanized steel destroys the zinc coating a n d requires a subsequent protective coating. A p plying the protective coating to u n avoidable welds in the beams was a headache. Celanese used wire 70 A
brushing, followed by one coat of a zinc-rich paint. T h e s e welds a r e in good shape today, b u t will probably need subsequent coats to maintain protection. A better, more per m a n e n t protection m a y be obtained by sandblasting followed by metalizing. In large-scale construction, plans for metalizing welds in gal vanized structures should be seri ously considered. Alternatively, weld treatment by tinning with a zinc solder is possible. Welding galvanized metal to light gage stainless steel tank walls can result in cracking in the tank walls, but this can be avoided by removal of the zinc coating before welding. T a n k s having cracked walls as a result of welding galvanized to stainless steel can be repaired by grinding out the cracks and filling with weld metal. Specifications to fabricators of stainless steel vessels who will use galvanized supports or strengthening rings should include a note to avoid any possibility of stainless steel embrittlement by zinc penetration.
Preconstruction G a l v a n i z i n g
Hot-dip galvanizing of structural steel before construction is a massproduction operation with low labor and material costs. Structural steel can be galvanized for about $50 per ton. H e l m (3) has reported gal vanizing costs as low as $0.17 to $0.33 per square foot for varioussized I-beams. T h e low cost of galvanizing is understandable, con sidering t h a t the zinc in a 2-ounce coating costs only $0.013 per square foot. At least two coats, and prob ably three, of paint are needed to provide protection to ungalvanized structural steel ; three coats of paint can easily r u n $0.075 per square foot for the paint alone. Obviously, less labor is needed to hot-dip steel before construction t h a n to paint it afterward. I n new construction, a great saving can be m a d e by gal vanizing whenever suitable, avoiding unnecessary high-cost coatings. T h e life of a galvanized article is directly proportional to the thickness of the zinc coating. However, a 2-ounce coating is the m a x i m u m practicable because of its tendency to flake in thicker coatings a n d its prob lems in application.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Protective Limitations
Galvanized structural steel is ex tensively used in the Houston indus trial area for protection against the h u m i d semimarine Gulf Coast at mosphere and industrial fumes, but its satisfactory use is limited to areas where corrosive acid and highly al kaline fumes are absent or negligible. T h e protective coat of zinc oxides which normally form on a galvanized surface are dissolved by such at mospheres, resulting in rapid attack of the zinc. Celanese found it unsatisfactory in a n acetic acid area in the Bishop, Tex., plant. I n moderately corrosive chemical at mospheres, some companies prefer to paint galvanized steel. M a i n t e nance costs on painted galvanized steel are less t h a n painted ungal vanized steel because surface prepa ration is simplified. T h e galvan ized coat protects the steel where cracks have formed in the old paint coat, m a k i n g extensive descaling unnecessary. In some severe chemi cal corrosion areas, cement-coated steel beams are used in preference to either galvanizing or painting. T h e r e is m u c h literature dealing with galvanized steel properties to aid those planning new construction. For those w h o wish to pursue the topic, several references are listed. Hughes (4) gives a most compre hensive bibliography on the field, a n d Burns and Bradley (2) discuss the production a n d properties of zinc coatings in considerable detail. References
(1) American Zinc Institute, Inc., 60 East 42nd St., New York 17, Ν. Υ., "How Zinc Controls Corrosion," 1955. (2) Burns, R. M., Bradley, W. W., "Pro tective Coatings for Metals," ACS Monograph 129, 2nd éd., Reinhold, New York, 1955. (3) Helm, F. P., Oil Gas J. 55, No. 14, 106-9 (1957). (4) Hughes, M. L., "Hot Dip Galvanizing —1946-1956," Sowell Publications Ltd., 4 Ludgate Circus, London E. C. 4, England.
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