NSF International Supplement - NSF Standard 14 Addresses Dezincification and Stress Crack Corrosion
At last year’s NSF 14 Joint Committee meeting, a task group was formed to discuss adding the test requirements of two standards related to dezincification and stress crack corrosion (SCC) to NSF/ANSI Standard 14, Plastics Piping System Components and Related Materials.
The reason: Under certain conditions, such as hard water, fittings and valves made from copper alloys containing more than 15% zinc by weight intended for potable water systems may become susceptible to failure due to either dezincification or stress crack corrosion.
The proposal is to utilize the methodologies and reference ISO 6509, Corrosion of Metal and Alloys - Determination of Resistance of Brass and ASTM B858, Standard Test Method for Amonia Vapor Test for Determining Susceptibility to Stress Corrosion Cracking ion Copper Alloys.
Here is an overview of dezincification and stress crack corrosion.
DezincificationDezincification is a condition associated with brass alloys in which zinc ions diffuse out of solution, leaving porous copper behind. Migrated ions can then accumulate at the surfaces, resulting in large deposits called meringue dezincification, which can lead to reduced internal pipe diameter and eventually blockage. In addition to deposits, the remaining porous copper can lead to water weepage and, in extreme cases, complete failure due to reduction in mechanical properties. This condition was first documented in marine applications during the World War I when condenser tubes would plug up from major deposits, taking entire ships out of commission. Later in the 1950s, dezincification was recognized in duplex (alpha-beta phase brass alloy) pipe fittings when many areas suffered wide spread blockages in hot water supplies. It was found that some geographic areas had a higher rate of incidents due to residual minerals in the water.
To prevent this condition, research began in the 1960s to develop a dezincification resistant (or DZR) brass alloy, as well as tests to evaluate the material’s resistance. In 1980, amendments to BS 2872 and 2874 were made to include composition, heat treatment, and DZR testing requirements for CZ 132 brass alloys. The testing procedure was then adopted as an ISO standard – ISO 6509 – in 1981 to evaluate all brass alloys. The test specified in ISO 6509 provides an effective means to evaluate the resistance of a brass alloy to dezincification.
To perform the testing, a piece of brass is mounted into a non-conducting plastic resin and cross-sectioned to expose approximately 100 mm2 of surface area. The specimens are then immersed in a 1% copper (II) chloride solution at 167ºF for 24 hours. After exposure, the samples are cross-sectioned perpendicular to the surface to reveal the dezincified region. Polished samples are examined using a metallograph to measure the depth of effected region and compared to the maximum depth called out in the standard.
Stress Crack CorrosionStress crack corrosion is a process in which the formation and propagation of cracks are accelerated by tensile stresses while in a chemically reactive environment. Such stresses can be residual from cold rolling, welding, machining or applied loads. Typically, SCC cracks are on the microscopic level, making the detection of compromised parts difficult.
To evaluate resistance to stress crack corrosion, the test method defined in ASTM B858 is utilized. In this test, an ammonia atmosphere is used to rapidly corrode samples, which may lead to the development and propagation of cracks. Although this method offers no correlation to a water environment, it is a standardized method that allows for comparison between tests already performed on currently marketed brass alloys.
To perform this testing, brass fittings are placed in a sealed test chamber filled with an aqueous solution of ammonium chloride. Sodium hydroxide is added to the solution to raise the pH to a level that mimics certain corrosive environments. Samples are suspended above the fluid level to allow the ammonium vapor to attack the metal surface. After 24 hours of exposure at room temperature, fittings are disassembled and all samples are immersed in a sulfuric acid solution to remove any corrosion products.
The fittings are then examined with a low power microscope for any signs of cracking. To examine the interior surface, samples are cut radially in half. Passing samples should not exhibit any signs of cracking. If there are questionable signs of cracking, the fitting may be bent or flatted slightly to open up any small cracks.
NSF 14 Moving ForwardAfter several NSF 14 Joint Committee meetings - where discussion was primarily focused on the validity of the test methods - a draft was adopted and presented to the 2009 NSF 14 Joint Committee as an issue paper. The committee agreed to send the draft to ballot. The proposal references both test methods. Also, it establishes sampling requirements, testing conditions and pass/fail criteria. It will require a minimum of three samples to be tested.
Testing, according to ASTM B858, will be done at a pH of 9.5. The pass/fail criteria for the ISO 6509 test method will be a maximum dezincification depth of 200 µm, while the pass/fail criteria for the ASTM B858 test method will be any evidence of cracking using a magnification of 10x.