GMW 14872—2006　Cyclic Corrosion Laboratory Test（）

1 Socpe
Note: Nothing in the standard supersedes applicable laws and regulations unless specific exemption has beeen obtained.

Note: In the event of conflict between the English and domestic language,the English language shall take precedence.

1.1  Purpose. This procedure describes an accelerated laboratory corrosion test method to evaluate assemblies and components. The test procedure provides a combination of cyclic conditions （salt solution, various temperatures, humidity, and ambient environment） to accelerate metallic corrosion. The procedure is effective for evaluating a variety of corrosion mechanisms, such as general, galvanic, crevice, etc. The test exposure/conditions can be individually tailored to achieve any desired level of corrosion exposure.

Also, synergistic effects due to temperature, mechanical and electrical cycling along with other stresses can be comprehended by this test. See Deviations Section, which after the Appendices, for typical modifications.

1.2 Foreword. The test method is comprised of 1%（approximate）complex salt mist applications coupled with high temperature and high humidity and high temperature dry off. One（1） test cycle is equal to 24h.

A cycle is made up of the daily events or test inputs illustrated in Appedix A,Figure A1. A cycle normally requires one （1） day to complete. The test exposure is dictated by a targeted coupon mass loss. A target rane for the number of cycles necessary to meet the required mass loss is provided in Appendix A, Table A1. The coupon mass loss values are used to verify that the correct amount of corrosion has been produced by the test. In addition to meeting mass loss requirements, the test must be conducted such that the number of cycles required to meet coupon mass loss requirements falls within the specified range.
 
1.3　Applicability. This is a cyclic corrosion test（refer to Appendix A, Figure A1） used for validation testing （cosmetic and/or functional/general durabiliy）, development testing, and quality control testing, for all global environmental regions （refer to GMW8738 for specific Method and applicable region）.

The basic exposures for cosmetic corrosion are found below in exposures A through C. Exposure D is used for functional assessment（refer to Appendix A, Table A1）.　All corrosion mechanisms are accelerated at different rates on test. Completing the test may not ensure that a component passes the functional requirement. A comprehensive failure mode analysis must be conducted to ensure all failure modes are considered and life expectancy is met.

Exposure A:Underbody Components
Exposure B:Underhood Components
Exposure C:Exterior Components/Panels, Secondary Surfaces, and Interior Components
Exposure D: All Components（Functional）
Note: Reference the Deviations section of this document for optional modifications.

2 References
Note: Only the latest approved standards are applicable unless otherwise specified.

2.1 External Standards/Specifications.
ASTM D610          ISO 8407
ASTM D1193         ISO 12103-1
ISO 6270-2         SAE J2329
2.2　GM Standards/Specifications.
GMW 8738           GMW 15357
GMW 14700          GMW 15358
GMW 15282          GMW 15359
GMW 15356
2.3　Additional References.
SAE 2001-01-0640

3 Resources
3.1　Facilities. Laboratory （preferably controlled ambient conditions）.

3.2　Equipment.
3.2.1 Ambient Stage. The apparatus for the ambient stage shall have the ability to maintain the following environmental conditions and per Appendix A, Figure A1:
Temperature: 25±3℃
Humidity: 45±10%RH
3.2.2　Humid Stage. The apparatus for the humid stage shall have the ability to maintain the following environmental conditions and per Appendix A, Figure A1:
Temperature: 49±2℃
Humidity: ～100%RH
The apparatus shall consist of a fog/environmental chamber, suitable water supply conforming to ASTM D1193 Type IV, provisions for heating the chamber and the necessary means of control.

3.2.2.1　Water Fog. The apparatus shall include provisions for a supply of suitably conditioned compressed air and one or more nozzles for fog generation. The nozzle or nozzlers used for the generation of the fog shall be directed or baffled to minimize any direct impingement on the test samples.

At least 2 clean fog collectors shall be placed within the exposure zone so that no drops of solution from the test specimens or any other runoff source shall be collected. The collectors shall be placed in the proximity of the test specimens, one nearest to any nozzle and the other farthest from all nozzles. Collections rates for each 80cm2 of horizontal collection area should be in the range of 0.75 to 1.5mL/h（on average） of water will be collected in each collector over a minimum duration of 16 h. Fog collection rates may be adjusted within this range as necessary to meet mass loss target rates.

Suitable collecting devices include glass or plastic funnels with the stems inserted through stoppers into graduated cylinders.Funnels with a diameter of 10cm have an area of about 80cm2. Where samples cannot be read immediay upon completion of the humid stage, closed cell foam balls can be used in combination with the collections funnels（i.e. foam ball in mouth of funnel） to allow moisture to collect while minimizing evaporation.

3.2.2 Wet-Bottom. The apparatus shall consist of the chamber design as defined in ISO 6270-2. During "wet-bottom" generated humidity cycles the tester must insure that visible water droplets are found on tha samples to verify proper wetness.

3.2.2.3 Steam Generated Humidity. Steam generated humidity may be used provided the source of water used in generating the steam is free of corrosion inhibitors. During steam generated humidity cycles the tester must insure that visible water droplets are found on the samples to verify proper wetness.

3.2.3 Dry off Stage. The apparatus for the dry off stage shall have the ability to obtain and maintain the following environmental conditions:
  Temperature:60±2℃
  Humidity:≤30%RH

The apparatus shall also have sufficient air circulation to prevent temperature stratification, and also allow thorough drying of the test samples.

3.2.4 Salt Mist Application. The solution shall be sprayed as an atomized mist, and should be sufficient to rinse away any salt accumulation left from previous sprays. The test samples and coupons shall be thoroughly wet/dripping. Suitable application techniques include using a plastic bottle, or a siphon spray powered by oil-free regulated air to spray the test samples and coupons.

No The force/impingement from this salt applocation should not remove corrosion or damage coatings/paints system of test samples.

3.2.5 Corrosion Coupons and Mounting Hardware. Coupons serve to monitor the average general bare steel corrosion produced by the test environment. Coupons consist of 25.4mm wide x 50.8mm long x3.18 mm thick pieces of bare SAE 1008-1010 carbon steel, cold rolled steel per SAE J2329 CR1E, uncoated, no post coating treatment, which are stamped with an alphanumeric identification number （reference Appendix A, Figure A2）.

The coupons shall be secured to an aluminum or nonmetallic coupon rack with fasteners as shown in Appendix A, Figure A3 and Figure A4. The bolt, nut and washers shall be made from a non-black plastic material, preferably nylon. Appendix A, Figure A4 shows a completed coupon rack configuration. The number of coupons recommended for different test durations are shown in Appendix A, Table A2.

3.3 Test Vehicle/Test Piece. Test samples shall be representative of production intent. Sample size shall be consistent with durability requirements determined by the appropriate Material/Corrosion Engineering department.

3.4 Test Time.
 Calendar time: See Appendix A, Table A1
 Test hours: See Appendix A, Table A1
 Coordination hours: See Appendix A,  Table A1