Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2001-02-16
2002-10-08
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S253000, C148S260000
Reexamination Certificate
active
06461450
ABSTRACT:
This invention relates to a process for controlling the layer weight during the phosphating of steel strip zinc-coated on one or both sides. By this means the layer weights may be reliably maintained within the required range of from about 1 to about 2 g/m
2
even when the belt speed, hence the phosphating time, is altered or when other phosphating variables are altered.
Whenever steel strip zinc-coated on one or both sides is mentioned for the present purposes, this means both electrolytically zinc-coated and hot-dip zinc-coated steel strip. It also means steel strips coated with zinc alloy. In the latter, the zinc layer contains additional alloying constituents, such as iron, nickel and/or aluminum.
The term “layer weight” is frequently used in the field of metal surface phosphating. Instead of “layer weight” or, more fully, “phosphate layer weight”, the terms “layer thickness” or “mass per unit area” are also used. By this is meant the mass per unit area of the metal phosphate layer produced on the metal surface by the phosphating. It is generally expressed in g/m
2
. It may be determined by weighing a phosphated metal sheet having a known surface area, removing the metal phosphate layer and reweighing the metal sheet. The mass of the metal phosphate layer per m
2
may be calculated from the weight difference found, taking into account the surface area of the metal sheet. A 0.5 wt. % solution of chromic acid, for example, may be used to remove the metal phosphate layer. The method for determining the layer weight is described in more detail in German Standard DIN 50942.
The layer weight is an important variable for controlling the results of the phosphating. Layer weights within different ranges are sought, depending on the intended use of the phosphated metal parts. The present invention is concerned chiefly with sheet metal used in the manufacture of automobiles. Here layer weights of more than 0.8 g/m
2
, but of about 4 g/m
2
at most, are sought. The layer weights are to be preferably less than 3 g/m
2
and in particular from about 1 to about 2 g/m
2
.
Processes for phosphating surfaces of iron, steel, zinc and alloys thereof, as well as of aluminum and alloys thereof are long-standing prior art. The phosphating of the aforementioned surfaces serves to increase the adhesive strength of paint layers and to improve the corrosion protection. The phosphating is carried out by dipping the metal surfaces into the phosphating solutions or by spraying the metal surfaces with the phosphating solutions. Combined processes are also known. Formed metal parts, such as car bodies, may be phosphated, as may also metal strips on high-speed production lines. The present invention is concerned with such a phosphating of strip. Phosphating of strip differs from phosphating of parts in that, because of the high conveyor belt speeds the phosphating, i.e. the development of a closed metal phosphate layer, has to take place within a short period of time, for example, about 2 to about 20 seconds.
Processes for phosphating metal strips, in particular steel strips which have been zinc-coated electrolytically or by hot dipping, are known in prior art. For example, WO 91/02829 describes a process for phosphating steel strip which has been zinc-coated electrolytically and/or by hot dipping; the process involves short-term treatment with acidic phosphating solutions which, besides zinc ions and phosphate ions, contain manganese cations and nickel cations as well as anions of oxygen-containing acids having an accelerating action. The latter definition is to be understood as referring in particular to nitrate ions. DE-A-35 37 108 likewise describes a process for phosphating electrolytically zinc-coated steel strips by treatment with acidic phosphating solutions which, besides zinc ions, manganese ions and phosphate ions, contain other metal cations, such as nickel ions and/or anions of oxygen-containing acids having an accelerating action, in particular nitrate ions. Here the zinc cation content is within the relatively low range of 0.1 to 0.8 g/l.
German Patent Application DE-A 196 39 596 attempts to provide a phosphating process which, on the one hand, solves the problem of pinholing and, on the other hand, also renders it possible, within the short phosphating times usual on production lines, to produce a closed crystalline phosphate layer on steel strips which are not zinc-coated on either side and on the zinc-free side of steel strips zinc-coated on one side only. By “pinholes” are meant whitish corrosion points on the metal surface which have a crater-like appearance in micrographs. Such pinholes are frequently formed on zinc-coated steel surfaces when the phosphating solution has too high a content of chloride ions and/or nitrate ions. According to the abovementioned document, this object is achieved by a process for the phosphating of steel strip, or of steel strip coated on one or both sides with zinc or zinc alloy, by spray treatment or dipping treatment for a period within the range of 2 to 15 seconds, using an acidic phosphating solution containing zinc and manganese, at a temperature within the range of 40 to 70° C., characterised in that the phosphating solution contains:
1 to 4 g/l zinc ions
0.8 to 3.5 g/l manganese ions
10 to 30 g/l phosphate ions
0.1 to 3 g/l hydroxylamine in free, ionic or bound form and not more than 1 g/l nitrate ions, and has a free acid content within the range of 0.4 to 4 points and a total acid content within the range of 12 to 50 points.
DE-A 197 40 953 describes a process for the phosphating of steel strip, or of steel strip coated on one or both sides with zinc or zinc alloy, by spray treatment or dipping treatment for a period within the range of 2 to 20 seconds, using an acidic phosphating solution containing zinc, magnesium and manganese, at a temperature within the range of 50 to 70° C., characterised in that the phosphating solution is free from nitrate ions and in that it contains:
1 to 4 g/l zinc ions
1.2 to 4 g/l manganese ions
1 to 4 g/l magnesium ions
10 to 30 g/l phosphate ions
0.1 to 3 g/l hydroxylamine in free, ionic or bound form, and has a free acid content within the range of 0.4 to 4 points and a total acid content within the range of 15 to 45 points.
If in addition nickel ions are added to the phosphating solutions containing zinc and manganese, the so-called “trication phosphating solutions” are obtained.
The terms “free acid” and “total acid” are generally known in the field of phosphating. They are determined by titrating a sample from the acid bath with 0.1 N sodium hydroxide solution and measuring the consumption of the latter. The consumption in ml is expressed as a number of points. In this document, the number of points of free acid means the consumption in ml of 0.1 N sodium hydroxide solution required to titrate 10 ml of bath solution which has been diluted to 50 ml with deionised water, to a pH of 4.0. Similarly, the number of points of total acid gives the consumption in ml to attain a pH of 8.2.
Various methods of adjusting the layer weight to within the required range are known in prior art. For example, this may be achieved by altering the belt speed, the other bath variables remaining the same. Usually, however, a certain belt speed is preset, so that the phosphating bath variables have to be adjusted in such a way that they produce layer weights within the required range at the preset belt speed. Here the belt speeds may vary considerably and they may range, for example, between about 20 and about 180 m/min. Possible ways of regulating the layer weight known hitherto are: altering the temperature of the phosphating bath, altering the free acid, the total acid and/or the concentration of the layer-forming ions. However, the response to these alterations is only very slow, so that it takes considerable time for layer weights within the required range to be obtained. In this connection, it is particularly inconvenient to adjust the layer weight by altering the composition of the bath. These alterations may often only be rever
Geruhn Dieter
Klare Andreas
Panter Frank
Peters Hubertus
Ricke Franz-Gerd
Harper Stephen D.
Henkel Kommanditgesellschaft fur Aktien
Oltmans Andrew L.
Ortiz Daniel S.
Sheehan John
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