Galvanised metal, for example, galvanneal, has a dull and rough surface appearance. The gaivanneal layer is of the order of 8-10pm thick and comprises several phases wherein the iron concentration decreases and the zinc concentration increases towards the surface of the layer. Adjacent to a steel substrate surface is a gamma phase (Fe5Zn2,) which is followed by a delta phase (FeZn7). The outer layer is predominantly a zeta phase (FeZn,3) but if under-alloyed, free zinc (Zn), or an eta phase, may be present.

It is possible to manipulate the ratio of the three main phases, while maintaining a constant concentration of iron, by changing the annealing cycle used in the post-galvanising process.

Zinc-iron galvanised steel sheet has excellent spot weldability, paint adhesion and corrosion resistance, but suffers poor formability due to a flaking phenomenon during pressing which is known as"powdering".

Studies have suggested that paint adhesion is dependent on alloy composition, with good adhesion properties observed with an iron content of less than 10%, marginal adhesion with an iron content of between 10 and 11 % and poor adhesion for an iron content greater than 11%. This correlates with an increase in more iron-rich and brittle phases of galvanneal.

X-ray diffraction and X-ray fluorescence studies have indicated that there is also a correlation between the composition of an alloy layer and the degree of powdering. Therefore, the ability to measure alloy composition and control it on galvannealling process lines will be extremely important.

Iron content is only one component of the phases present in the galvanneal layer and detailed phase information is required to fully understand the relationship between composition and properties such as powdering. For example, the molten zinc bath used in the galvanising process contains aluminium which is also known to affect coating phase composition and properties. Some studies have indicated that an aluminium concentration in the coating of less than 0. 14% offers good resistance to powdering.

The gamma phase of the galvanneal layer is known to be the hardest.

Therefore, it has been suggested that powdering will be reduced most effectively by minimising the thickness of the gamma phase. For good coating properties, some experts believe that the gamma phase should comprise less than 5% of the total thickness. However, this theory is not universal because of the complexity of factors which contribute to the final formability of the steel. For example, higher bath aluminium concentrations delay the formation of the gamma phase and for given anneaiing conditions produce thinner layers.

It is, however, widely agreed that coatings consisting mainly of a zeta phase exhibit lower incidences of powdering. The zeta phase has a low hardness and resists cracking by relaxing compressive stresses by deforming itself. Unfortunately, this property leads to higher friction during pressing and hence poor drawability.

Table I shows the properties exhibited by different phases in the galvanneal layer. Hence, the powdering behaviour of the galvanised product must be minimised by finding a balance between phase layers that lies between the limits of over- and under-alloying. Under-alioying causes stickiness as a result of the zeta and eta phases at the coating surface, whereas over-alloying results in hardness and brittleness at the surface.

TABLE I Phase FormulaFe(wt%)Charactenstics Eta Zn 0 very ductile Zeta FeZn13 5-6.2 ductile DettalFeZr)-7-11.5brittfe Gamma 1 FesZn21 15.8-19.8 hard and brittle Gamma Fe3Zn,0 20. 5-28 brittle It is an object of the present invention to provide a means to analyse the surface layer of a ga ! vannea ! coating in order to control the ga ! vanneahing process and provide an optimum coating which reduces or substantially eliminates the problems associated with powdering.

Accordingly, in one aspect the invention provides a method for obtaining phase information from the surface of galvanised metal comprising the use of laser ablation and optical emission spectrophotometry and/or mass spectrophotometry.

It is known that laser ablation may be used as a method of removing small samples from metal surfaces. Control of the laser power density and the focus spot diameter enables the sample volume to be controlled. Each laser pulse produces a discrete plasma which emits wavelengths characteristic of the atom or ion population produced from the ablated specimen. The intensities of the element-specific wavelengths may be directly related to composition using a conventional optical emission spectrophotometer.

In one example, pulsed lasers with wavelengths ranging from the ultraviolet to the infra-red spectrum may be used, that is, between 10-6 and 104 cm Alternatively, or additionally, the laser ablated material may be transferred as an aerosol using an inert carrier gas to a secondary excitation source, such as a mass spectrophotometer, for qualification. Laser ablation will fragment the metal sample into positive, negative and neutral fragments.

A mass spectrophotometer will deflect the fragments in a strong magnetic field in which the fragments will be deflected according to their mass-to- charge ratio. Analysis of the spectrum produced provides detailed information about the composition of the laser ablated material.

In another aspect, the invention provides a method for obtaining phase information from the surface of galvanised metal comprising the use of laser ablation to excite atoms/ions at the surface and optical emission spectrophotometry to identify the atoms/ions so excited.

In yet another aspect, the invention provides a method for obtaining phase information from the surface of galvanised metal comprising the use of laser ablation to remove a sample of atoms/ions at the surface, collecting the sample and analysing the sample by mass spectrophotometry.

Typically, depths of sample ablated from the surface are as little as 0.3,um, preferably between 0. 1 and 0. 2nom, for example 0.15pm.

Preferably, the galvanised metal is galvanised steel, that is galvanneal.

In another aspect, the invention provides apparatus for identifying phase information on the surface of galvanised metal by the use of laser ablation and optical emission spectrophotometry and/or mass spectrophotometry.

In yet another aspect, the invention provides a method for controlling the physical properties of galvanised metal by altering the composition of the galvanised layer, including the steps of : - (a) identifying phase information on the surface of galvanised metal with the use of laser ablation and optical emission spectrophotometry and/or mass spectrophotometry ; and, (b) adjusting annealing cycles to produce the required composition.

It is to be understood that the foregoing is merely exemplary of the invention and that various modifications can be made thereto without departing from the true scope of the invention as set out in the claims.