Materials Chemistry最新文献

An important difference between metallic and nonmetallic coatings is that the former can significantly affect the corrosion behaviour of the coated metal at sites where the coating is incomplete or damaged whereas generally organic coatings have no such effect. Metallic coatings afect the behaviour of the basis metal because an electrolytic cell is established in which either the basis or the coating metal will act as an anode and corrode at an enhanced rate. When the coating is the anode the basis metal will form the cathode and usually will corrode at a lower rate than in the complete absence of the coating. However, it is well known that simple considerations of anodic and cathodic behaviour do not explain all the phenomena involved in the corrosion and protection of metal coated metals. Other factors of importance include the formation of insoluble corrosion products and the influence of time and temperature on this process, the electrochemical polarisation behaviour of the metals, and the nature of the corrosive or inhibitive environment. The uses and properties of some types of metal coatings will be described in relation to these factors and the implication of these to test methods discussed. One particular system viz. galvanised steel in water containing a corrosion inhibitor will be described in more detail in terms of electrochemical behaviour.

In order to provide further evidence concerning the reliability of predicting relative protective performances of metal organic coatings by using electrochemical testing methods, some experimental results, obtained with such an approach, are shown.

The methods are based on Ecorr/time, polarization and polarization resistance measurements on samples immersed in a corrosive medium.

The results concern with different type of metallic materials coated with electrophoresis techniques.

Adhesion, hardness, abrasion resistance data are also given; the comparison between traditional checking tests and the electrochemical ones is discussed.

The corrosion resistance of chromate layers on zinc sheet was studied by electrode impedance measurements in 0.1 mol/1 NaClO₄ solutions. A correlation was found to exist between the inhomogeneity and corrosion protection of the chromate layers and impedance data measured at different spots of the specimen. The results of impedance measurements and humidity tests agreed within the experimental error.

A brief survey is given of electrochemical test methods available for testing the protective properties of paint and other organic coatings over metals in corrosive environments, so as to establish the reasons for choosing A/C impedance as a method for both basic research and routine testing work.

A description is given of the technique and the apparatus used. For research purposes, a transfer function analyser (TFA) is used to apply a small amplitude a/c signal to a cell containing the painted test panel in a corrosive liquid (3.5% aqueous sodium chloride solution as standard). The apparatus automatically programmes a frequency sweep between 10 KHz and 10 mHz and analysesthe response and presents the results as Nyquist or Bode plots. Typical results for studies on a series of epoxy resin paints are presented. Results are also presented showing the use of a commercially available A/C impedance meter for use as a routine testing instrument, as well as studies on damaged paint systems (scribed paint panels).

Finally, an assessment is given of our experience with A/C impedance as both a background research and a routine testing technique for monitoring corrosion of metals under paint films.

The dissolution of galvanized steel in acid solution was followed by thermometric, potential-time and weight loss techniques. In HCl and H₂SO₄, only the zinc coat is attacked, while in HNO₃, curves representing stepwise dissolution of the coating layer and steel base are obtained. The time, t, required for complete dissolution of the zinc coat as measured by the three techniques is comparable.

Dissolution of steel pieces of different coating thickness has been examine in 1 M H₂SO₄ by the thermometric method. Starting from the initial temperature, T_i, t, then temperature of the system rises to attain a maximum value, T_m, after a time, t, then decreases. Up to 6 g/dm² coating thickness, both ΔT(T_m − T_i) and t vary linearly with coating thickness. The results suggest the thermometric method for the determination of coating thickness of galvanized steel.

The selection and development of electrochemical test methods for the assessment and improvement of protective properties of coatings are performed currently at CSM.

Examples of application of electrochemical test methods for the evaluation, research and development of tinplate for food containers and of painted steel sheets for car bodies are given.