Subject and scope this document describes the neutral salt spray test, which allows evaluation of the corrosion resistance of metallic materials protected or unprotected by permanent or temporary coatings.This standard need a climatic chamber--salt spray test chamber.
However, it does not allow a priori relationship between the strength observed during testing and the corrosion resistance of the various media in which the material is available.Under no circumstances is this test a salt spray test.The performance of various materials in the test process should not be regarded as an indicator of their relative corrosion resistance in use.
The purpose of this method is to describe the salt spray corrosion test of the relevant equipment and to carry out the necessary verification to ensure the conformity of the test.This method can be applied to samples of laboratory samples, parts or vehicle parts.
The saline solution percentage must be 5%±0.5%.
The sodium chloride must not contain, in the anhydrous state, more than 0.2% total impurities and more than 0.1% sodium iodide. It must be free from nickel and copper.
The distilled water or de-ionised water must not contain more than 0.02% impurities. Its PH must be 7±1. Its conductivity must be less than 0.2ms/m ( measurements to be made lees than 6 hours before use).
Dissolve 5 prats of sodium chloride by weight into 95 parts of distilled water. Check the concentration by measuring the density of the solution at 35℃±1℃. Carry out this check each day. The density of the 5% solution must be between 1030 and 1040 kg/m³. Adjust the PH of the solution to the value 7.0. Before spraying, eliminate if required the impurities in suspension by filtering or decanting.
The air must be pure, maintained at 85-90% relative humidity at a temperature of 35±1℃ and delivered to the sprayers at a pressure of 0.1Mpa at an accuracy of 20%.
In order to purity it, pass it through a water filter.
In order to maintain a constant concentration of the saline solution, humidity the air at a higher temperature than 35℃ by making it pass in finely divided bubbles through a saturator containing water heated to a suitable regulated temperature. The height of the column of water is less important than the fineness of the bubbles since saturation of very fine bubbles is almost instantaneous. The water in the saturator must be changed each week to eliminate impurities.
The salt spray is defined by the characteristics of the solution gathered in the collectors during the test.
The intensity of the spray must be such that for each 80cm² of the horizontal collecting surface 2ml±1ml of solution is gathered per hour, on the basis of a minimum operating time of 16 hours.
The solution gathered must have the density and the PH should be 7.0.
The dimension and the method of construction of the spray chamber are left to the discretion of the constructors and the users, on the condition that the following arrangements are observed:
The walls of the chamber, the frame and the supports situated on the inside must resist the salt spray corrosion. Among the materials which exhibit good resistance are: glass, rubber, certain plastic materials,cement.
The design of the chamber must be such that the spray can be deposited directly by weight onto the parts or specimens. To this end, a suitable arrangement is being studied for the sprayers and the orifices for evacuating the condensed liquid.
The design of the walls of the chamber, the framework and supports must be such that the liquid which runs over the surfaces cannot flow onto the parts or specimens. The condensed solution is evacuated at the bottom of the chamber without being re-used.
With a view to facilitating the rationalisation of the equipment, a recommended type of chamber is shown with its principal dimension.
Use one or several compressed air sprayers. A diagram in appendix 2 describes a sprayer of this type as an example.
Preliminary tests allow the determination, once and for all, of the inclination to be given to the deflector in relation to the axis of the jet and its distance from the sprayer to obtain the most uniform distribution of spray possible, controlled by the quantities of solution gathered in the different collectors.
The heating device must be able to maintain the interior of the spray chamber at a temperature of 35℃±2℃.
Different methods may be used:
It is desirable that the air penetrates into the spray chamber at a temperature higher than 35℃. The degree of excess is controller by:
The maintenance of a temperature of 35℃ inside the chamber.
The thermal capacity per unit mass of the walls and the ambient temperature.
The volume of pulsed air
The air pressure, which determines the temperature necessary to obtain the required humidity.
The temperature is between 43℃ and 47℃ for a pressure between 0.08Mpa and 0.12Mpa ( 0.8 and 1.2bar)
It is desirable, in general, that the ambient temperature around the spray chamber should be as uniform as possible. To this end the chamber may be placed in a room at constant temperature, or the chamber may be surrounded by an envelope containing water at a suitable temperature. The chambers which are completely insulated may be heated with warm air. However this method may require the use of an automatically controlled source of auxiliary heating, allowing the temperature to be raised rapidly after opening the chamber.
It is practically impossible to satisfy the temperature characteristics by the use of heating elements immersed in the reservoir of the saline solution.
The device for measuring the temperature inside the chamber must allow either continuous inspection or a routine of two inspections a day. One may therefore use either a continuous recording device inside the chamber or a thermometer where the reading may be effected from the outside. The reading of the temperature must be carried out with the chamber closed in order to avoid, when opening, condensation on the thermometer bulb which could lead to errors in reading.
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