Does hydrostatically testing an aluminum-lined composite cylinder with tap water corrode the cylinder or make the cylinder unsafe?

No. Tap water of varying quality from all around the world has been used to hydrostatically test and retest aluminum and aluminum-lined composite cylinders for more than 30 years without harming cylinders or making them unsafe to use—provided, of course, that the cylinders have been properly dried after testing, as required by regulation. The worldwide safety record for some three million aluminum-lined composite cylinders is excellent, which affirms that tap water is an appropriate hydrostatic test liquid. Moreover, test agencies have an excellent record of properly drying cylinders before releasing them for service. In short, based on Luxfer field experience and user feedback, tap water corrosion of aluminum and aluminum-lined composite cylinders in the field is not a problem.