US 4,358,887 AGranted

Method for galvanizing and plastic coating steel

OwnerJohn A. Crepsinventor

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Abstract

A method and apparatus for galvanized pipe and other formed sections which sometimes include reentrant angles and strapping. The pipe and other sections are sized and hot dipped in a bath of molten zinc at a temperature of about 850.degree. F. (450.degree. C.) and then controlled for thickness with an air knife. The molten zinc is cooled at a rate of 20.degree.-70.degree./second by an air blast or an air amplifier to solidify the zinc coating before the formation of more than 0.00008" to 0.00018" of intermetallic compound to form a layer of zinc from 0.0004" to 0.0008" in thickness. The zinc is water quenched without the formation of spangles and plastic coated with a layer of polyesters, vinyl alkyds or fluorocarbons, from 0.002" to 0.006", resulting in a coated steel that does not develop extensive rust corrosion in less than 1000 hours in a neutral salt spray. The formed product meets ASTM A-525 specifications and is designated G-30 or G-60 according to the thickness of the zinc layer.

Claims13

Dependent claims

A method of galvanizing and plastic coating pipe and tubular sections, which comprises the following steps in sequence:

(a) feeding a coil of cold rolled or hot rolled ferrous metal strip to a forming mill;
(b) looping and welding the ends of the coil to produce an endless strip;
(c) cleaning the strip to remove contaminants;
(d) forming a tube section in a forming mill and seam welding said section;
(e) sizing the section to finished dimension;
(f) cleaning the strip to remove contaminants;
(g) heating said pipe sections to galvanizing temperature;
(h) galvanizing the sections with molten zinc at about 850.degree. F.;
(i) controlling the zinc thickness with an air knife under variable pressure;
(j) accelerated cooling the molten zinc to solidify it with air blasts at a rate of 20.degree. F.-80.degree. F./second so that the molten zinc is held in the range of about 750.degree. F.-850.degree. F. for a period of 5-30 seconds between the steps of galvanizing and water quenching and so that the zinc alloy layer after said step of quenching is 0.0018" or less in thickness to prevent the formation of spangles;
(k) water quenching the sections to solidify the zinc completely;
(l) plastic coating the pipe;
and
(m) pulling said formed sections through the foregoing processes.

11 more elements

Dependencies are inferred from the claim text.