EP1029111B1 - Corrosion protection for galvanised and alloy galvanised steel strips - Google Patents

Abstract

A process for the corrosion protection of steel strips coated with zinc or zinc alloy, characterised in that the steel strips coated with zinc or zinc alloy are brought into contact with an aqueous treatment solution having a pH within the range of from 1.5 to 3.5, which contains 1 to 20 g/l manganese(II) ions and 1 to 150 g/l phosphate ions, and the solution is dried without intermediate rinsing. Optionally the solution may contain in addition: up to 10 g/l zinc ions, up to 10 g/l nickel ions, up to 20 g/l titanium ions, up to 50 g/l silicon ions in the form of silicon compounds, up to 30 g/l fluoride ions, up to 150 g/l of one or more polymers or copolymers of polymerisable carboxylic acids selected from acrylic acid, methacrylic acid and maleic acid, and esters thereof with alcohols having 1 to 6 carbon atoms. The present invention also relates to the correspondingly-treated metal strips.

Description

The invention relates to a new method for corrosion protection of galvanized and galvanized steel strips. On the one hand, the method causes one Temporary corrosion protection for transport and storage purposes. The term "Temporary corrosion protection" should be understood to mean that the metal surfaces are effectively protected against corrosion for transport and storage times, until they are covered with a permanent anti-corrosion layer such as a Paint to be coated. On the other hand, the method according to the invention serves as Pretreatment of the metal surfaces before painting, immediately after the application of the method according to the invention to the metal surfaces can be done.

As a measure for temporary corrosion protection, galvanized or alloy-galvanized steel strips are either simply oiled or phosphated or chromated if higher corrosion loads are expected. However, these measures are not sufficient in the case of particularly high corrosive loads such as ship transport in a salty sea atmosphere or storage in a tropical environment. The best temporary corrosion protection measure known in the prior art is chromating, in which the metal surfaces are coated with a layer containing chromium (III) and / or chromium (VI) with a layer coating of generally about 5 to about 15 mg / m ² Chrome plated. Because of the known toxicological problems of chromium compounds, this process is disadvantageous and complex from the aspects of occupational safety, ecology and the necessary disposal. In addition, chromated sheets are not very suitable for subsequent phosphating because on the one hand they lead to a chromium load on the cleaning solutions and on the other hand the metal surfaces cannot generally be phosphated over the entire area. Phosphating as an alternative measure to temporary corrosion protection can undesirably change the appearance of the metal surfaces. In addition, phosphating is complex in terms of plant technology, since depending on the substrate material it requires an additional activation step and, as a rule, a passivation step after the phosphating step. Passivation is often carried out using chromium-containing treatment solutions, which also results in the disadvantages mentioned above of using chromium-containing treatment solutions.

The invention has for its object a method for protecting against corrosion to provide galvanized and alloy galvanized steel strips that is less ecologically problematic and technically easier to implement than the aforementioned corrosion protection measures. This new process is supposed to the conventional methods with regard to paintability or paint adhesion be at least equivalent, but corrosion protection for storage purposes improve.

US-A-4,264,378 describes a chromium-free phosphating process, the Metal surface with an acidic, aqueous phosphate-containing solution that a pH of 1.5 to 3.0 is treated. This solution contains a divalent or higher cation as well as molybdate, tungstate, niobate or Tantalate ions. Coming as divalent or higher cations for example Ca, Mg, Ba, Al, Zn, Cd, Fe, Ni, Co and / or Mn in question. The Solution can also be finely divided silicon or film-forming organic polymers, as they are common in paint manufacturing, for example polyacrylates. The treatment solution is on the metal surface without rinsing dried up.

In contrast, the present invention relates to a method for the corrosion protection of galvanized or alloy-galvanized steel strips, characterized in that the galvanized or alloy-galvanized steel strips are brought into contact with an aqueous treatment solution with a pH in the range from 1.5 to 3.5, which releases is of soluble molybdate, tungstate, vanadate, niobate and / or tantalate ions and the 1 to 20 g / l manganese (II) ions, 1 to 150 g / l phosphate ions and up to 150 g / l of one or more polymers or copolymers of polymerizable carboxylic acids selected from acrylic acid, methacrylic acid, maleic acid and their esters with alcohols having 1 to 6 carbon atoms
contains, and the solution dries without intermediate rinsing.

Accordingly come as substrate materials for the method according to the invention electrolytically galvanized or hot-dip galvanized steel strips in question. The Steel strips can also be alloy galvanized, i. that is, an electrolytic or in Apply hot-dip zinc alloy layer.

The most important alloy components for zinc are iron, nickel and / or Aluminum. The thickness of the zinc layer or the zinc alloy layer lies here usually in the range between about 2 and about 20 microns, in particular between about 5 and about 10 microns.

The applied treatment solution is dried without intermediate rinsing. Methods of this type are known in the art under no-rinse methods or dry-in-place methods. The treatment solutions can be sprayed onto the metal surfaces or applied by passing the steel strips through the treatment bath. The desired amount of the treatment solution remaining on the metal surface, which leads to the desired layer coverage of 1 to 5 g / m ² , can be adjusted by squeezing rollers. However, it is advantageous to apply the treatment solution directly in the desired layer support by means of a roller system, as is known, for example, as a "chemcoater".

The treatment solution for the process according to the invention contains 1 to 150 g / l, preferably 10 to 70 g / l phosphate ions. The phosphate content is here as Phosphate ions calculated. However, the person skilled in the art is aware that the pH to be set according to the invention in the range from 1.5 to 3.5 is only a very high low proportion of the phosphate is present as triple negatively charged phosphate ions. Rather, there is a balance of free phosphoric acid, primary and secondary phosphate ions, which is determined by the acid constants of phosphoric acid for the different protolysis levels and depends on the specifically chosen pH value. Most of the phosphate is in the free pH range Phosphoric acid and as primary and secondary phosphate ions.

bis zu 10 g/l, vorzugsweise zwischen 2 und 4 g/l Zinkionen,

bis zu 10 g/l, vorzugsweise 3 bis 6 g/l Nickelionen,

bis zu 20 g/l, vorzugsweise zwischen 3 und 7 g/l Titanionen, die vorzugsweise als Hexafluorotitanationen eingesetzt werden,

bis zu 50 g/l, vorzugsweise zwischen 15 und 25 g/l Silicium in Form von Siliciumverbindungen wie beispielsweise Hexafluorosilicationen und/oder feindisperse Kieselsäure mit einer mittleren Teilchengröße unterhalb von 10 µm,

bis zu 30 g/l Fluoridionen, die als freies Fluorid in Form von Flußsäure oder von löslichen Alkalimetall- oder Ammoniumfluoriden oder in Form von Hexafluoroanionen von Titan oder Silicium eingebracht werden können. Freies Fluorid wird, unabhängig davon, ob es als freie Säure oder als lösliches Salz in die Lösung eingebracht wird, bei dem einzustellenden pH-Wert der Behandlungslösung als Gemisch von Flußsäure und freien Fluoridionen vorliegen.

The corrosion protection to be achieved with the method according to the invention can be further improved if the treatment solution additionally contains one or more of the following components:

up to 10 g / l, preferably between 2 and 4 g / l zinc ions,

up to 10 g / l, preferably 3 to 6 g / l nickel ions,

up to 20 g / l, preferably between 3 and 7 g / l titanium ions, which are preferably used as hexafluorotitanate ions,

up to 50 g / l, preferably between 15 and 25 g / l silicon in the form of silicon compounds such as, for example, hexafluorosilicate ions and / or finely dispersed silica with an average particle size below 10 μm,

up to 30 g / l fluoride ions, which can be introduced as free fluoride in the form of hydrofluoric acid or soluble alkali metal or ammonium fluorides or in the form of hexafluoro anions of titanium or silicon. Free fluoride, regardless of whether it is introduced into the solution as a free acid or as a soluble salt, is present at the pH of the treatment solution to be set as a mixture of hydrofluoric acid and free fluoride ions.

The treatment solution also contains up to 150 g / l, preferably between 60 and 125 g / l of one or more polymers or copolymers polymerizable Carboxylic acids selected from acrylic acid, methacrylic acid and maleic acid and their esters with alcohols with 1 to 6 carbon atoms. If here Generally speaking of "treatment solution", it means that the organic polymers, depending on the type, also as a suspension in the active ingredient solution can be present. The same applies here, depending on the acid constants used Carboxylic acids at the pH value of the treatment solution as a mixture of free acids and acid anions. Especially it is preferred to use one or more polymeric carboxylic acids together at least one of the optional components zinc, nickel, Use titanium, silicon and fluoride.

Manganese and, if desired, zinc and nickel can be more water soluble in the form Salts, for example as nitrates, are introduced into the treatment solution.

However, it is preferable not to introduce any other foreign ions into the treatment solution other than the above-mentioned components. Manganese, zinc and nickel are therefore preferably used in the form of the oxides or carbonates, so that they are ultimately present in the treatment solution as phosphates. It is also preferred to use titanium, silicon and fluoride in the form of hexafluoro acid. At most, the silicon can be introduced in the form of finely dispersed silicas, which preferably have a specific surface area in the range from 150 to 250 m ² / g.

The temperature of the treatment solution on contact with the metal surface should preferably in the range from about 20 to about 40 ° C. Lower temperatures slow down the reaction rate and lead to an increasing weaker corrosion protection, higher temperatures shorten the Exposure time due to drying too quickly and are therefore also disadvantageous. After an exposure time that depends on the belt speed, in the case of spray or Diving application from the length of the treatment zone and in any case from the distance between the treatment zone and the drying device and the in is usually between 1 to 6 seconds, the treatment solution is through Dried up temperature increase. This can be done by Metal surfaces are treated with infrared radiation. However, it is easier to use the Treatment solution to drive wetted metal strips through a drying oven. This should have such a temperature that the object temperature, that is Temperature of the metal surface, ranging from about 60 to about 120 ° C established. In Anglo-Saxon usage, this object temperature is also called "Peak Metal Temperature" called.

embodiments

The invention was tested on test sheets made of hot-dip galvanized steel. Since these were covered with an anti-corrosion oil, they were first cleaned with a commercially available alkaline cleaner. In the practical application of the method in belt systems, however, the cleaning step can be omitted if the method according to the invention immediately follows the process of galvanizing or alloy galvanizing. The treatment solutions according to Table 1, which had a temperature of 20 ° C. and a pH of 3.2, were applied to the sample sheets by immersion and adjusted to a wet film thickness of 6 ml / m ² by centrifuging at 550 revolutions per minute , The wetted sheets were then dried in a circulating air drying cabinet set at 75 ° C. Drying cabinet temperature and drying time resulted in an estimated peak metal temperature of 70 ° C.

The sample sheets pretreated according to the invention and untreated or chrome-plated comparison sheets were subjected to a constant alternating climate test according to DIN 50017 and a salt spray test according to DIN 50021 SS. The number of days until the sample sheets reached a visually estimated level of rust 5 was recorded. The results are shown in Tables 2 and 3. Bath compositions (g / l in deionized water) component Beisp.1 Beisp.2 Beisp.3 Mn 3.2 5.4 9 phosphate 6 10 91 Zn - - 3 Ni - - 3 Ti (as H ₂ Ti F ₆ ) 5 8th - Si (as SiO ₂ dispersion) - - - Acrylic acid polymer 125 75 75 Constant alternating climate test according to DIN 50017: days to rust level 5 treated with solution untreated chromated Beisp.1 Beisp.2 Beisp.3 1 7 > 142 > 142 > 142 Salt spray test according to DIN 50021 SS: days to rust level 5 treated with solution untreated chromated Beisp.1 Beisp.2 Beisp.3 <0.5 6 7 7 10

Claims (4)

1. A process for protecting galvanized or alloy-galvanized steel strips against corrosion, characterized in that the galvanized or alloy-galvanized steel strips are contacted with an aqueous treatment solution which has a pH of 1.5 to 3.5, is free from soluble molybdate, tungstate, vanadate, niobate and/or tantalate ions and contains

   1 to 20 g/l manganese(II) ions,

   1 to 150 g/l phosphate ions and

   up to 150 g/l of one or more polymers or copolymers of polymerizable carboxylic acids selected from acrylic acid, methacrylic acid, maleic acid and esters thereof with C_1-6 alcohols

   and the solution is dried without intermediate rinsing.
2. A process as claimed in claim 1, characterized in that the treatment solution additionally contains one or more of the following components:

   up to 10 g/l zinc ions,

   up to 10 g/l nickel ions,

   up to 20 g/l titanium ions,

   up to 50 g/l silicon in the form of silicon compounds,

   up to 30 g/l fluoride ions.
3. A process as claimed in one or both of claims 1 and 2, characterized in that the treatment solution has a temperature of 20 to 40°C.
4. A process as claimed in one or more of claims 1 to 3, characterized in that the treatment solution is dried after a contact time of 1 to 6 seconds at a substrate temperature of 60 to 120°C.