Coating processes – With pretreatment of the base – Heating or drying pretreatment
Reexamination Certificate
1998-06-30
2001-08-21
Beck, Shrive (Department: 1762)
Coating processes
With pretreatment of the base
Heating or drying pretreatment
C427S234000, C427S310000, C427S329000, C427S433000, C427S436000, C034S105000, C034S107000, C034S217000
Reexamination Certificate
active
06277443
ABSTRACT:
FIELD OF INVENTION
This invention relates generally to a batch process for making galvanized steel products with a low lead or no lead galvanized coating, and more specifically, to a galvanizing process in which metal parts are treated in an aqueous pre-flux solution and then thoroughly dried before they are immersed in a molten zinc bath that is low lead or no lead.
BACKGROUND
Galvanized coatings are commonly applied for corrosion protection of metal parts, and especially steel or iron parts. In the galvanizing process, a solidified layer of zinc is formed on a part's surfaces by immersing the part into molten zinc which contains approximately 1% lead. The main benefit of lead's presence in zinc is that it makes the process of galvanizing less demanding and less sensitive to many unfavorable circumstances, like insufficiently pickled, cleaned and even rusty steel surfaces, the absence of preheating and even drying when wet parts are immersed in molten zinc, and so on.
Unfortunately, lead cannot now be used for galvanizing steel parts coming into contact with drinking water, since small but significant quantities of lead from the galvanized parts dissolve in the water. Such dissolved lead may be accumulated in human and animal bodies with very deleterious results. Consequently, a number of state legislatures have passed laws which, since 1995, forbid the presence of lead in galvanized products which contact drinking water.
Attempts have been made to reduce the lead level from conventional levels, for example, containing a maximum of about 1.4% by weight lead in Prime Western grade zinc to a High Grade Standard requirement level of a maximum of about 0.03% by weight lead, or to a Special High Grade Standard requirement of a maximum of about 0.003% by weight lead (ASTM B6-87 standard). However, attempts to reduce lead to less than about 0.5% by weight of lead have resulted in insufficient wetting of steel parts and finished products having a high percentage of uncoated surfaces, that is, black or bare spots.
Other techniques have been developed in the galvanization process in conjunction with the use of lead to produce high quality galvanized coatings. Metal parts are usually treated with aqueous solutions prior to immersing the parts into the galvanizing bath. Metal parts typically undergo alkaline cleaning, rinsing, acidic pickling, and rinsing treatments. After the parts have been cleaned, rinsed, pickled and rinsed, they are usually immersed in a pre-flux solution which is an aqueous solution containing ammonium chloride or a mixture of ammonium chloride and zinc chloride. The application of pre-flux has been found to remove metal oxides, for example, iron oxide from steel part surfaces, to promote good wetting with the molten zinc.
A top flux is often employed in conventional processes. Top fluxes are typically comprised of the same ingredients as a pre-flux, except that the salts such as zinc chloride and ammonium chloride are molten in form and float on top of the galvanizing bath. Top fluxes have the further advantage that they reduce or eliminate metal spattering when steel articles are immersed into the galvanizing bath, which can occur if the article is still wet with aqueous solutions such as pre-flux.
As a metal part having a surface temperature substantially lower than the molten zinc is immersed in the molten zinc bath, a frozen layer of zinc is formed on the surface of the part. The frozen zinc layer forms before the molten zinc is able to adequately wet the surface of the steel, and hence there is poor adhesion of the frozen zinc onto the metal part surface. However, soon thereafter, the frozen layer on the part surface gradually melts. When the layer is totally melted, the metal surface is wetted by molten zinc and growth of a zinc-metal intermetallic alloy forms an integrated surface between the zinc and the metal part.
Perhaps the most significant benefit of lead is that it accelerates the melting of the frozen zinc layer on the surface of the part, and galvanization takes place more rapidly. This phenomenon is described in U.S. Pat. No. 5,437,738. Lead also significantly reduces surface tension of molten zinc and increases its fluidity. This results in better wetting of the steel surface to be coated and higher drainage after withdrawal of the parts from the kettle. Another technique that has been used to accelerate the rate of galvanization, is to preheat the parts after the parts are immersed in pre-flux. Preheating is typically conducted at high temperatures, for example, greater than 200° C., so that the steel surfaces are wetted by molten zinc as rapidly as possible. Also, parts that are arranged in a batch have many contact surfaces with adjacent parts and require additional heat for drying. Since conventional pre-fluxes are decomposed or burned when heated, for example, to 200° C. to 250° C. for 3 to 15 minutes, a non-conventional, heat-stable pre-flux is needed.
It is desirable to provide a new galvanizing process that is low-lead or no lead for producing uniform, void-free coatings on metal parts. It is desirable to provide a galvanization process that essentially thoroughly dries the parts after the parts are immersed in a conventional pre-flux solution. It is also desirable to provide a galvanization process in which the top-flux treatment can be eliminated. It is desirable that the galvanizing process can be carried out in batch operation. In addition, it is desirable to provide a low-lead or no-lead galvanizing process with galvanizing baths that may contain low concentrations of aluminum to promote adhesion and a bright finish to the galvanize coating.
SUMMARY OF THE INVENTION
The invention herein provides for a low-lead or no-lead batch galvanization process that produces void-free zinc coatings on metal parts. The molten zinc bath contains about 0.1% or less, preferably, about 0.05% or less, more preferably, about 0.005% or less, and even more preferably, about 0.003% or less by weight lead. Metal parts to be galvanized typically undergo one or more of the cleaning, rinsing, pickling, and pre-flux treatments in aqueous solutions, the aqueous solutions being well known in the art. According to the invention herein, the process comprises applying a pre-flux solution having a temperature of at least about 65° C.; drying the surface of the parts such that the surface of the parts are essentially dry, and the surface temperature of the parts is maintained at about 125° C. or less; and applying molten zinc to the parts to form a galvanized coating on the parts. The wet parts are dried, preferably, by placing them in a dryer system that passes hot, dry air over the surface of the parts while the parts are rotated.
In one embodiment of the invention, a plurality of steel pipe, arranged in a batch, are treated in a series of aqueous solutions. The pipe members are preferably washed, rinsed, and then dipped in pre-flux solution having a temperature of at least about 65° C. and comprising ammonium chloride in water. Next, the batch of pipe members are placed in a dryer system and each pipe is essentially, thoroughly dried such that the surfaces of the pipe achieve a maximum temperature of about 125° C. The pipe members in the batch, while in the dryer system, are rotated in a motion relative to one another by a rotating system during drying.
The rotating system preferably comprises a descrambler system and more preferably, both a descrambler system and a divider apparatus. The descrambler system rotates a batch of wet pipe to the dryer system that comprises a hot, dry air source. The pipe members are arranged such that the surface of each pipe is in contact with the surfaces of adjacent pipe. As the descrambler system rotates the batch of pipe within the dryer system, the individual pipe rotate along their longitudinal axes. The pipe rotate relative to one another, and as they rotate, their original contact surfaces become exposed to dry air when they establish new contact surfaces with adjacent pipe.
Even more preferably, as the descrambler
Barr Michael
Beck Shrive
Calfee Halter & Griswold LLP
John Maneely Company
LandOfFree
Low lead or no lead batch galvanization process does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Low lead or no lead batch galvanization process, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Low lead or no lead batch galvanization process will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2537663