The invention relates to a method of providing a zinc alloy coating on a steel tube in a continuous processing line wherein the steel tube is passing through a liquid zinc alloy bath. It also relates to a steel tube provided with a zinc alloy coating according to the method.

Zinc alloy coated, or galvanised steel tubes are well known in the art and are used in a wide range of applications, for instance in central heating systems, industrial packaging, automotive parts, greenhouse constructions, roof constructions, recreational equipment, fencing systems and household appliances.

The corrosion resistance of zinc or zinc alloy coated steel is considered sufficient for many applications. However, it has appeared in practice that, in particular in saline environments, the corrosion resistance sometimes does not meet the quality requirements of the customer.

The addition of magnesium to a zinc based coating in order to improve corrosion resistance is known in the art and trials with such a zinc-magnesium based coating showed improved results with respect to zinc based coatings without magnesium. For these trials a zinc based coating alloy was used with a 1.4 to 1.8 % (w/w) magnesium content and a 1.4 to 1.8 % (w/w) aluminium content, with the remainder being zinc and unavoidable impurities. The addition of aluminium also contributes to the increase of the corrosion resistance of the coating. This is a well-known coating alloy composition which e.g. is used in the automotive industry and has excellent anti- corrosion properties.

Although the corrosion resistance improved considerably compared to the zinc based coating alloys without magnesium, the use of magnesium in the coating composition gives rise to another problem. These coatings show a certain degree of darkening, staining or discolouration of the coating over time. This is a problem, in particular for applications wherein the steel tubes are clearly visible, such as in household appliances or use in automotive parts where aesthetic properties play an important role.

The inventors, who do not wish to be called superficial in any sort of way, tried to find solutions for both problems. It is therefore an objective of the present invention to provide a steel tube with a coating with improved anti-corrosion properties. It is another objective to provide a steel tube with a coating that shows no or minimal staining or discolouration over time. Accordingly, a method of providing a zinc alloy coating on a steel tube in a continuous processing line is provided wherein the liquid zinc alloy bath comprises 0.10 to 0.40 % (w/w) magnesium, 0.05 to 0.14 % (w/w) aluminium and unavoidable impurities, the remainder being zinc. With this zinc coating alloy applied, the corrosion resistance is increased considerably and at the same time the staining or colouration of the coating over time and / or when exposed directly to open air remains at a low level. In the trials it appeared that the normally applied upper limit of the magnesium content could be lowered to 0.10 to 0.40 % (w/w), without severely affecting the corrosion resistance. With an aluminium content in a range of 0.05 to 0.14 % (w/w) aluminium it appeared that for most applications togetherwith the magnesium content of the alloy, a sufficient corrosion resistance is provided and at the same time a minimal degree of staining or colouration is achieved. Therefore these results were considered to have an optimal balance between the two requirements. Preferably the aluminium content is in the range of 0.10 to 0.14 % (w/w) aluminium. More preferably the aluminium content is 0.10 % (w/w) aluminium.

Good results have also been realised by providing a zinc alloy coating that comprises 0.20 to 0.35 % (w/w) magnesium, and with a zinc alloy coating that comprises 0.25 to 0.35 % (w/w) magnesium. A lower amount of magnesium has the added benefit, whilst still protecting against corrosion, that the hardness of the coating layer increases compared to magnesium-free coatings. This results in better scratch and wear resistance when used in practice. Preferably the zinc alloy coating comprises 0.30 % (w/w) magnesium.

Preferably the zinc alloy coating has a thickness in a range of 4 to 25 pm. Typically the thickness is in a range of 6 to 25 pm and a thickness in a range of 12 to 25 pm is suitable for most applications. This gives enough corrosion protection while still giving a good appearance.

Preferably the zinc alloy coating is provided only on the outside of the steel tube. When used for sanitary piping or the like these tubes should not have a zinc containing coating on the inside.

If the steel tubes provided with the coating are used in very corrosive conditions, it could happen that despite the zinc alloy coating, after some time, red rust occurs. According to a preferred embodiment, to prevent corrosion and / or colouration in such harsh conditions, a top coating layer is provided on top of the zinc alloy coating. Such a top coating can either be a transparent layer or a coloured layer, wherein the top coating layer is an organic coating layer. Preferably, a urethane, acrylate or epoxy based organic coating layer is used as top coating layer. The organic coating can be applied as a dispersion of small particles of non- water soluble organic polymer binders in water (having a pH between 4 and 5) to steel tubes in-line by spraying, dipping or wiping. After applying the organic coating, the organic coating layer is cured and / or dried in-line before the steel tube is cut to length, for instance by means of an induction or infrared heating device.

Such coatings deliver increased corrosion protection, no white rust due to storage, protection against colouration / staining, better tube bendability (less tooling friction) and finger print resistance. The organic coating is applied with a thickness that is typically in a range of 1 to 5 pm and preferably in a range of 1 to 3 pm and is also referred to as thin organic coating, or TOC.

To optimise the investigation of corrosion resistance together with staining resistance a matrix of different coating samples were prepared in the laboratory with a Rhesca zinc hot dip simulator with varying concentrations of both aluminium and magnesium. See table 1 below.

Table 1.

For each bath chemistry, at least 5 samples were produced. For corrosion testing, one sample of each bath chemistry was chosen having the best properties, i.e. one side a layer thickness between 9 and 11 pm and free of bare spots. For staining tests, samples were chosen having a uniform surface. Coating thickness was not considered to be of importance. All samples were exposed to a staining test, by dipping into a 0.1 M solution of acetic acid at room temperature for 20 seconds. All samples were drawn out of the solution with minimal drag-out and rapidly dried by a blast of cold air, avoiding stray droplets as much as possible. That way, the surface retained its uniformity (no dried droplets). No staining occurred during this procedure. Exposure of the dried but not rinsed samples to humid and warm atmosphere (80 % relative humidity at 40 °C) for 4 days (96 hours) and 15 days (360 hours) caused a decrease of L-values measured with the Specular Component Included (SCI) method using a Minolta CM2002 spectrophotometer. This measurement method measures all light reflected from the substrate, including the light reflected at the angle of incidence. A decrease of the SCI L-value is an indication that less light in total has reflected from the surface after exposure of the acid covered samples to humid atmosphere, i.e. darkening has occurred during the exposure. This darkening is thought to be due to local chemical erosion of the surface by electrochemical corrosion, leading to surface roughening. The corrosion products formed are not black, but the roughening of the metal surface is thought to prevent reflection of light that enters the pits, increasing light absorption.

Table 2 shows the results of staining, percentage red rust after 504 hrs of salt spray testing and appearance of white rust after 1008 hrs VDA 233-102 cyclic testing (a cyclic corrosion test of materials and components). Light colour“yes” means the sample has not darkened appreciably as seen by the naked eye, whereas light colour “no” means that the sample showed a clear darkening as seen by the naked eye. “Coarse” means large amounts of porous white rust deposits and“fine” means white- wash type of appearance having a low amount of deposit.

Table 2.

In this test, it is clear that the L-value of those samples not containing any Mg hardly decrease in this test, indicating no staining with time. The L-values decrease with more magnesium and longer exposure times to humid atmosphere (before and right after dipping all samples did not show any decrease of L-value). The high aluminium grades were seen to be less sensitive to staining with the addition of magnesium than the other samples.

The values in table 2 show clearly that magnesium leads to staining, and more magnesium leads to more staining. Probably, a higher amount of aluminium (5 %) makes the coating less sensitive to staining (more gloss retention in case of high magnesium content). Even a minor addition of magnesium is strongly beneficial, which is very fortunate in the light of reducing staining issues.

The VDA-test after 1008 hours of testing (6 weeks), shows some red rust specks on the Zn1.6AI sample only. All samples acquire a whitish appearance, But all samples not containing any magnesium show coarse and repulsive deposits of white oxide.