Лако-красочные материалы — производство

This technique has rarely been used in adhesive bonding studies, but is well suited to determine the environmental degradation of adhesively bonded systems. Currently it is frequently used to study the degradation processes of painted metals immersed in an electrolyte, such as 5% Nacl [94]. Essentially the technique measures the total impedance of a sheet metal coated with a film of high resistivity. The metal is immersed in the electrolyte, held at its corrosion potential, and a small AC voltage perturbation is applied. The current response is measured and plotted as impedance |Z| versus the frequency. The usual spectrum is a plot of this impedance versus frequency over the range 0.1-105 Hz (Bode plot), although there are other formats as well. The spectrum is recorded at several intervals, for instance once a week or month.

From the Bode plot the total capacitance of the coating system and the total resistivity of the system can be determined directly from the graph by extrapolation to zero frequency. With the use of a so-called equivalent circuit, a hypothetical electrical circuit that would give the same response over the frequency range, the capacitance and resistivity can be broken down into the individual components for the metal, the interface, and the coating [95,96]. Parameters such as coating capacitance, double-layer capacitance, pore resistance, and polarization resistance can all be derived from the appropriately chosen model. These can then be converted to physical properties such as percentage of water uptake, diffusion rates of ions and liquid water through the coat­ings, numbers of pores, corrosion reactions (anodic and/or cathodic) that take place at the interface, blister resistance (related to adhesion of the coating), and so forth. These parameters are normally determined as a function of immersion time so that the best system of metal pretreatment and type of coating can be determined and further optimized quickly and in the laboratory without having to rely on lengthy exposure in outdoor tests.

Excellent correlations between EIS analyses and field exposure of painted metals have been reported [97]. The coatings can be very thick (such as paint systems or adhe­sives), but very thin films, such as oxide, plasma coatings, conversion coatings, or other pretreatments, can also be investigated. As an example, Fig. 17 shows EIS curves as a

function of immersion time in 3% NaCl of a cold-rolled steel sample coated with a film of plasma-polymerized trimethylsilane [98]. The variable in this study was the plasma clean­ing procedure applied to the metal prior to film deposition. Table 5 shows a comparison of the parameters that can be derived from the impedance data as a function of pretreatment using an equivalent circuit consisting of two resistances and two capacitors plus the resistivity of the electrolyte. It is seen that all parameters show the same ranking order and that order is identical to that observed in actual corrosion exposure. This example thus exemplifies that EIS data are quick and reliable and can be obtained in a fraction of the time required for actual exposure in a corrosion test. Further, the results of this study indicate that the pretreatment of the metal is by far the most important step in the plasma polymerization process. The actual composition of the deposited coating or its thickness is of secondary importance [98].

EIS and other electrochemical methods appear to be useful for the study of the performance of metal pretreatments (cleaning processes, anodization, phosphating, chro — mating, etc.) prior to adhesive bonding. A quick comparison of methods can be achieved, and because the method is fast and straightforward, it can be used as a quality control method. On adhesively bonded system EIS could be performed in more fundamental studies that would provide information on the nature and locus of degradation processes, when immersed in aggressive solutions.

galvanized steel [100]. Residual elements such as carbon can be detected below the coating. Also, entire conversion coatings can be profiled rapidly, such as phosphates and chromates or anodization layers, which are beyond the scope of AES or SIMS depth profiling. The technique could thus be used in quality control, for instance for determining the homo­geneity of electrodeposits across the width of the strip, or for control of the oxide thickness in metal pretreatment operations.

The technique requires only a moderate vacuum and has a spatial resolution of a few millimeters. Currently, two commercial instruments are available. Most elements can be detected, including hydrogen. Limitations are the quantification, which requires suitable standards, and the sample has to be electrically conductive. Nonconductive samples can be analyzed only by grinding them and mixing them with a metallic or graphite powder.

As a typical example, Fig. 18 shows the depth profile of a several microns thick metallic coating on steel [100].

small fraction of the primary particles. Below the surface, inelastic energy losses occur also. Therefore, the mass scale can also be converted to an energy scale. These principles are schematically shown in Fig. 19.

In addition to providing a depth profile, RBS can also give crystallographic infor­mation if the instrument is equipped with a precision goniometer. In certain directions of single crystals the phenomenon of channeling occurs. The backscatter yield is reduced when the beam is aligned with the major low index axes [104]. By aligning both incident and reflected beams along such directions, the relative positions of surface atoms can be determined with high precision. Also, the sites of impurity atoms can be determined, because for interstitial positions the backscatter yield is the same as for the random direction value.

H+, 4He+, other light ions 1-3 meV

~0.5-1.0 mm (—2 pm with microbeam)

-2-20 nA —5-30 min

-1-40 pC (6×1012-2.5×1014 ions)

170°

Surface barrier detector 15-25 keV energy resolution —1-2 pm

20-30 nm (3-4 nm with tilted targets)

Isotope resolution up to —40 amu 10~2-10~4 monolayers for heavy surface impurities Ю^-Ю-2 monolayers for light surface impurities 3-5% (typical)

RBS can be considered an almost ideal tool for the analysis of thin films of a few microns thickness if stoichiometric information is required. All elements >H are detected. Quantitative measurements of film thickness and composition and concentrations of impurities can be performed. A single RBS spectrum can show the amount of an impurity and its distribution throughout a thin film.

In general, however, only rather simple structures and compositions can be investi­gated successfully. The depth range that can be accessed varies but is of the order of 1-2 pm for proton and alpha particle beams. This depth depends on the primary particle energy, which has an upper limit of a few meV above which nuclear reactions may occur. The depth resolution is very high, e. g., 10-300 A, and can be improved for very low depths by tilting the sample.

Table 6 summarizes some of the parameters of RBS and Fig. 20 shows an example of the quantitative measurement of the epitaxial growth of an oxide film [101].