GMW 14872—2006　Cyclic Corrosion Laboratory Test（复合盐雾腐蚀试验）（二）

GMW 14872—2006　Cyclic Corrosion Laboratory Test（复合盐雾腐蚀试验）（二）

4 Procedure
4.1 Preparation.
4.1.1 Salt Solution Preparation. The comples salt solution in percent （%） by mass shall be as specified below:
  Sodium Chloride（NaCl）:0.9%
  Calcium Chloride（CaCl2）:0.1%
  Sodium Bicarbonate （NaHCO3）:0.075%
Sodium Chloride must be reagent grade or Morton Culinox 999 Food grade. Calcium Chloride must be reagent grade. Sodium Bicarbonate must be reagent grade（e.g., Arm & Hammer Baking Soda or comparable product is acceptable）. Water must meet ASTM D1193 Type IV requirements.

Note: Either CaCl2 or NaHCO3 material must be bissolved separatedly in water and added to the solution of the other materials. If all slid materials are added dry an insoluble precipitate may result.
Salt solution makeup calculator for the appropriate amount of sodium chloride, calcium chloride, sodium bicarbonate, and water examples are shown in Appendix B, Figures B1 through B3.
Additional contaminants（dust, grit, poultice, and exhaust condensate） called out in the Deviation Section are defined in Appendix C through Appendix F.

4.1.2 Coupon Preparation. Corrosion coupons should be cleaned with methanol or acetone solution and accuray weighed prior to use. The weight, in grams（g）, shall be recorded and retained for future reference. If coupons are not used immediay they should be stored such that they are corrosion free at the start of test.

It is critical that all forming or preservation oils/lubes be removed prior to exposure to allow for general/uniform corrosion of the coupon. This process can be aided by using a commercial grade degreaser prior to methanol or acetone clean.

4.1.3 Coupon Rack Preparation. Prior to Start of test; prepare the coupon rack with sufficient coupons to monitor the test. The number of coupons recommended for different test durations are shown in Appendix A, Table A2.

The exact location of each coupon on the rack shall be identified and recorded using the prestamped numbers for reference as illustrated in Appendix A, Figure A4.

Allow a minimum 5 mm spacing between the coupons and the rack surfacee.  All coupons shall be secured vertically with no more than 15 degree deviation from vertical and must not contact each other.

The coupon rack shall be placed in the general vicinity of the test samples being tested, such that the coupons receive the same environmental exposure as the test samples.

Note: Coupons are test monitoring devices and should not be exposed to additional stresses which may be added to the base test（i.e.,gravelometer, dust, grit, exhaust condensate, thermal exposure, etc.）

4.2 Environmental Conditions.
4.2.2 Test Conditions. Deviations from the requirements of the procedure shall have been agreed upon by test requestor. Such requirements shall be specified on part drawings, test certificateds, reports, ect.

4.3 Instructions. See Appendix A, Figure A1（Flow Diagram） for the steps that comprise the test method. Repeat the cycle daily, as necessary, until the test exposure requirements are met. At the option of the test requester, the test can be continued throughout weekends to decrease the over all test time provided that the number of cycles and mass loss requirements are met.

4.3.1 For each salt mist application, use the spray apparatus to mist the samples and coupons until all areas are thoroughly wet/dripping. The quantity of spary applied should be sufficient to visibly rinse away salt accumulation left from previous sprays. The first salt mist application occurs at the beginning of the ambient stage. Each subsequent salt mist application should occur approximay an hour and a half after the previous application in order to allow adequate time for test samples to dry.

Note: If graveometer exposure is required, test samples（not coupons）may be exposed to gravelometer testing per GMW 14700 before or during test exposure.

4.4 Test Acceleration. Humidity ramp times between the ambient and wet condition and between the wet and dry conditions can have a significant effect on test acceleration （this is because corrosion rates are highest during these transition periods）. Typically, the time from ambient to the wet condition should be approximay 1 h and the transition time between wet and dry conditions should be approximay 3h. These ramp times can be adjusted to increase or decrease test acceleration in order to meet targeted mass loss. Test acceleration can be optimized and tracked by using standard corrosion coupons as monitoring devices. Ramp time is to be included as part of the specified exposure period.

4.5 Test Monitioring. Corrosion coupons shall be removed and analyzed after a predetermined number of cycles （typically 5） throughout the test to monitor the corrosion （less frequent for longer exposures that is ＞40 cycles）. To analyze coupons remove 1 coupon from each end of the rack, clean to prepare for weighing, and an average mass loss determination.

4.5.1 Before weighing, clean the coupons using a mild  sand/bead blast （80±10 psi） process to remove all corrosion by-products fom the coupon surface. Wipe the coupons free of grit and weigh to determine the coupon mass loss using the formula:
Mass Loss = （Initial Mass） - （End-of-Exposure Mass）.

Note: Although not preferred corrosion by-product removal by chemical cleaning per ISO 8407 may be used.

4.5.2 Compare the actual mass loss to the targeted value. Refer to Appendix A, Table A1 for targeted mass loss values, in grams, for various test exposures as a function of the coupon's original thickness. Testing should be conducted as necessary to achieve necessary coupon mass loss. The number of cycles required to achieve required mass loss must meet that defined in Appendix A, Table A1.

Coupon mass loss targets corresponding to incremental test exposures are not included in Appendix A, Table A1. The processes defined in section 4.5 "Test Monitoring" and section 5.3.1"Coupons" may be used to check test progress and assure that the test is being run correctly. Corrosion mass loss should increase consistently between documented exposure values. If the actual mass loss does not fall within the targeted range for the specified exposure（s） as listed in Table A1 then the test should be repeated. Also, the reasons why the test did not fall within the target range should be investigated and corrected before resuming the test.

4.6 Cosmetic Corrosion Inspection. The test sample（s） shall be inspected for corrosion by means of GMW 15356, GMW 15357, GMW 15358, GMW 15359, and photographed（as necessary） at the end of predetermined cycles. Samples may be rated in terms of % corroded area per ASTM D610 or some other comparable standard if agreed upon by test requestor and tester. If test samples containing plastic and tester. If test samples containing plastic materials are being tested any discoloration or degradaton and/or adhesive bond failure（delamination） shall be noted and recorded.

If scribing is required, on test samples, follow the method described in GMW 15282. This method includes measurement of corrosion creepback from a scribe line. This method should be used when reporting test results unless stated otherwise on drawings or agreed upon by test requestor and tester.

4.7 End-of-Test Functional Inspection. At the end of test the samples shall be rinsed with fresh tap water and allowed to dry before evaluating. End-of-test functional analysis may involve sectioning, microscopic analysis and/or removal of corrosion product to determine degree and extent of base metal attack.