How To Do Salt Spray Corrosion Test?

The coating corrosion laboratory test method is based on the parameters of the field correlation laboratory test method and is determined by the SAE Automotive Corrosion and Corrosion Protection Association and the Automotive/Steel Cooperative (A/SP) Corrosion Working Group when designing the test process. The results obtained from this test provide a correlation with excellent coating corrosiveness in severely corrosive site environments.

This method was developed using a typical automotive coating system. If different types of coating systems are used, the relevance to the site must be determined.

The SAE J2334 laboratory test method is suitable for coating corrosion testing for special coating systems, substrates, processes or designs because it is a field correlation test and can therefore be used as an effective means and a development tool. In addition to measuring coating or general corrosion by using this method, if the corrosion mechanism is determined, the correlation with the site must be determined.

Definition

1.Coating Corrosion ---- Corrosion due to coating failure or breakdown. Typically, this corrosion is not caused by impact collision behavior but corrosion occurs.

2.General Corrosion ---- A typical bare (no organic coating) corrosion. Corrosion is essentially uniform and distributed over a large area.

3.Scored Body Corrosion ---- Corrosion of the painted substrate due to undercutting and corrosion at the scribed line. Scribes are controlled based on simulated failure points to represent nicks and cuts.

4.Compare Corrosion Specimens ---- Cold-rolled steel specimens experimentally controlled by weight loss and correlated with laboratory tests.

5.Experimental Controls ---- Samples (ie, samples, compare samples, specimens, etc.) that have been pretested and/or of known correlation. They can be used to control test performance and compare test results (and also to help evaluate repeatability and reproducibility).

Instruments And Test Materials

1.Chamber—The ability of a chamber to achieve and maintain the following environmental conditions (see SAE J1563, ASTM D 1735, and ASTM D 2247):

a. 50°C±2°C, 100% humidity (water mist/condensation)—water mist/condensation humidity can be obtained by using water spray, steam (steam) generator, etc. The test specimens and compare specimens should be visibly damp/wet.

b. 60℃±2℃, 50%±5%RH. Additionally, equipment capable of maintaining 50% relative humidity conditions is required. The air must circulate sufficiently to avoid temperature gradients and to dry the specimens during the drying procedure of the test cycle. Air circulation can be obtained by fans and forced convection.

2. Salt solution - A saline solution is a complex mixture of 3 salts, as follows:

0.5%NaCl

0.1%CaCl2

0.075%NaHCO3

NOTE 1—CaCl2 or NaHCO3 must be dissolved separately in deionized water (see ASTM D 1193, Section IV) and then added to the solution of the other materials. If all solid material is added simultaneously in the "dry" state, an insoluble precipitate may result. If a small amount of precipitate still occurs and these solutions are used in a spray device, it is necessary to remove the precipitate to avoid clogging the nozzle (ie filter or siphon the solution). Do not attempt to dissolve these precipitates by adding acid.

NOTE 2—Measure and record the pH of the saline solution prior to the test and weekly thereafter (see ASTM E 70-90). Do not attempt to adjust the pH with buffers of any kind.

The test specimens are immersed in the saline solution for 15 minutes at every other test cycle. Other methods of applying saline solution, such as manual spraying or direct generation of thin mist in the test chamber, meet the requirements, but this has not been analyzed for correlation, reproducibility or repeatability effects. 15 minutes (continuously) of fully wetting the part using a fully wet spray device should be sufficient. However, high intensity (pressure) sprays (impact test specimens) should be avoided. If a spray or mist is used instead of immersion in the application of the saline solution, it must be demonstrated that the results are equivalent to immersion or that they are simply an A:B relationship. Further research into the relationship between spray and brine techniques is ongoing. When these technologies become available, the results will lead to more advanced experimental methods. It is recommended to change the saline solution weekly and to stir the solution before using it.

Procedure

Cyclic test

Consists of 3 basic stages

1). Wet stage - 50°C and 100% humidity for 6 hours

2). Saline application stage - continuous operation for 15 minutes under room ambient conditions

3). Drying stage - 60°C and 50%RH for 17 hours and 45 minutes

The test cycle is repeated daily. The fully automatic test chamber can be placed normally on weekends or in a dry phase (it is commonly used to choose to run on weekends and holidays to complete the test early). This rule is an objection if compared to other laboratories that do not have fully automated capabilities (for manual operations, weekends are usually kept in dry conditions unless a 7-day operation is employed). The duration of the entire trial and the conditions over the weekend must be collated in the trial results. If two or more laboratories are testing similar parts, it is recommended to develop unchanged/common weekend test conditions before the start of the test. The transition time between the application of the saline solution stage (2) and the drying stage (3) belongs to the drying time. Similarly, the transition time between the drying phase (3) and the wet phase (4) belongs to the wet phase. The transition time should be recorded at each trial phase.

Evaluation of coating corrosion that is susceptible to damage. Test specimens are marked and then exposed (see ASTM D 1654). The length of the thread should be at least 2 inches. Corrosion of the scribed body is measured at predetermined intervals according to the specified corrosion class. The orientation of the scribing on the sample must be specified and documented (for typical plate samples, it is recommended that the plate be 15 degrees to the perpendicular line so that the samples do not obscure each other and the scribing line crosses diagonally on the surface of the plate).

Duration of the experiment

When evaluating coating products, SAE J2334 has a minimum of 60 cycles. Observing the difference in the performance of heavy metal coatings requires a long test time. Different test times should be determined based on other factors such as materials, influence of corrosion mechanisms, or historical history.

Specimen monitoring

A.The control corrosion specimen consists of 25.4 (wide) ×50.8 (long) rectangular bare steel (AISI-1010)) sheets. These control specimens are used to monitor the average corrosion of ordinary bare steel produced in the test environment.

B. Each control specimen should be printed with a number on the surface as a permanent marking.

C. The control corrosion specimen should be thoroughly cleaned and the methanol flushed with a suitable commercial degreaser to remove impurities and store oil/lubricating oil. It is then weighed, accurate to milligrams, and recorded for later reference.

D. The control specimen should be fixed to an aluminum or non-metallic control specimen holder, with fixtures and gaskets made of non-black plastic, preferably nylon, electrically isolated from the sample holder.

E. At least 5mm space between the control specimen and the sample holder surface. All control specimens should be positioned at least 15 degrees from the perpendicular and should not touch each other.

F. The control specimen holder should be placed near the test specimen so that the control specimen receives the same environmental exposure.

G. In the process of testing, after reaching the predetermined test cycle, the control sample should be taken out for analysis and monitoring the corrosion situation. When analyzing a control specimen, a control specimen is taken from the end of the control specimen holder and prepared for weighing and weight loss. Ensure that there are enough control specimens for exposure tests so that frequent corrosion monitoring can be accomplished. In addition, to accumulate data, control specimens without exposure can be added to the test to obtain batch data.

H. Before weighing, clean the control specimen with light blowing sand (preferably fine glass sand) to remove corrosion products from the surface of the control specimen. There is also an alternative/equivalent cleaning method, the chemical method, as described in ASTM G1. Once cleaned, wipe the control specimen with methanol and weigh it, using Equation 1 to determine the mass loss:

Mass loss = starting mass - mass after exposure (Equation 1)

Data Reporting

Compare specimen

The mass loss of the control specimen should be measured and recorded after each set of predetermined cycles (e.g., every 20 cycles). These are cumulative data. In addition, if necessary, unexposed samples can be placed and removed after the next set of cycles in order to obtain phased data.

Test samples

The corrosion width or corrosion rate of the scribed substrate of the test sample is measured and recorded at predetermined intervals (e.g. 20 cycles – only under rinsing conditions). At the end of the test, two scribing basic corrosion widths are recorded (if the coated sample is to be evaluated), one under rinse conditions and one after scraping and tape testing (see 1989 SAE Automotive Corrosion and Protection Conference Paper P228, pp. 144-5). As a guideline, the average, maximum, and minimum values (total width) of the corrosion width of the scribing substrate are recorded.definition

1. Total Width of Substrate Corrosion—a measure of the distance (perpendicular to the intersection of the scribe line) between the unaffected areas on either side of the scribe line of paint film, expressed in mm. (Loss of adhesion between coating and substrate).

2. Average value—the average value of the total corrosion width of the substrate measured at points distributed equidistantly with the dashed line as the center.

3. Maximum value—the total width of substrate corrosion measured at the point of greatest loss of adhesion, subtracting the dimension at the end of the scribed line.

4. Minimum - the total width of substrate corrosion measured at the point where the loss of adhesion is minimal, minus the dimensions at the end of the dash.

Test chamber

The test chamber shall have supporting materials and include the following information:

A test chamber

B Humidity

C temperature

D Wetting process

E Dehumidification process

F Heating process

G cooling process

H Air circulation process

I size

J capacity

K calibration process

I calibration cycle

M transition time between the two phases

TIPS

1. Saline change frequency (weekly or faster if impurities are uncertain)

2. Method of applying saline

3. PH measurement method

If a logger is used, the cycle diagram should be given along with the test sample data. If a recorder is not used, the written record should indicate the transition time between typical steady-state and steady-state conditions.

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