The answers are in context of PEM and alkaline electrolysis operating at or below 30 bar and below 85 deg C°. A general suggestion: Ask component suppliers about material compatibility, but do an independent investigation to confirm. As a general resource,  safety data sheets (SDSs) sometimes provide material compatibility information. Specific recommendations follow. 

• Hydrogen: Hydrogen material compatibility information can be found at Material Compatibility Hydrogen Tools (h2tools.org), including the very detailed technical reference developed by Sandia National Laboratories.
• Alkaline Water Electrolysis Systems: Cell stack electrolytes are typically potassium hydroxide, sodium hydroxide, or sodium chloride solutions. The pumps, piping, gas/liquid separators, and other components must be compatible. An example of an MSDS that provides information about material compatibility can be found at ERCO Worldwide: Potassium Hydroxide Solution.  Other resources include publications by the National Association of Corrosion Engineers and the Materials Technology Institute.
• Oxygen: Oxygen compatibility is a big concern, especially at pressure. ASTM subcommittee G04.02 affords no-cost access to apt standards and cleaning practices for oxygen. Start here: ASTM International Jurisdiction of G04.02. Other resources include CGA G-4.4, Oxygen Pipeline and Piping Systems. Only certain materials are rated for pressurized oxygen. Cleaning to remove particles and oils is very important to reduce fire hazards - remember, almost anything can be fuel in oxygen. 
• Water: Pure water feed to electrolyzers is important. A good approach is to consult with the water purification equipment supplier for recommended materials for the feed water supply components. High purity water corrosion products can contaminate PEM membranes and degrade electrolyte. 
• The use of plastic tubing in H2 and O2 pressure applications is usually precluded. See the AICHE CHS H2 Laboratory Safety course, which discusses a PNNL laboratory incident. Metal tubing is preferred. While plastic tubing may be desirable for non-conductivity and flexibility, one should only consider plastic tubing after a full hazard analysis to assure there are effective protective safeguards (e.g., ventilation, flow limits, protective enclosures, active leak detection, isolation/depressurization) in place. 
• H2 and O2 gases dissolve in significant quantities in liquids at 30 bar. Materials in these services will need to be compatible with the gas as well as the fluid. Note that these gases will readily come out of solution when pressure is reduced and directed to a drain. Open drains in well-ventilated areas are strongly recommended.

Pay particular attention to material compatibility of safety devices, such as pressure relief valves and pressure sensors. It is important to follow the guidance for proper design of vent systems given in CGA G 5.5 for H2 and EIGA Doc 154 for O2. These standards cover topics such as where back pressure is to be avoided and safe vent locations.
 

Cylinders are required to be tested periodically to verify structural integrity. The most common test method is hydrotesting, which means water is the most likely impurity that solidified in the inlets. Drying the tanks is normally a requirement after performing a hydrotest. In the USA, CFR 49 CFR § 180.209 applies: https://www.law.cornell.edu/cfr/text/49/180.209. Paragraph (b)(1)(v) requires that the “cylinder is dried immediately after hydrostatic testing to remove all traces of water.” This requirement is vague since there is no definition of “all traces” and might be interpreted to simply mean no visible water rather than a small enough quantity to meet the 99.9995% purity requirement. If the problem is caused by the gas supplier not meeting the specified purity requirements, there are some approaches that can be used for quality assurance of gas purity: 

1. Discuss the issue with the supplier and put in contract language that a significant fraction or possibly all cylinders be tested and certified for purity. 
2. Use non-steel cylinders since steel is usually the worst material for this issue. 
3. Purchase gas from a supplier that verifies the moisture within cylinders after requalification. 
4. Use a liquid nitrogen cold trap to remove water from the gaseous hydrogen flowing into the process.

It's important to note that over long periods of time, even the impurities within high purity systems can accumulate. It’s also important to understand that many suppliers will guarantee the total purity spec (i.e., percentage) based upon the measured impurities. Other impurities may exist, but not be included in the total purity, if not included on the list of impurities to be tested. In hydrogen, one of the most frequent impurities is helium since it is difficult to detect and it can pass through many hydrogen production and purification processes. However, it’s usually considered to be benign to nearly all processes. Helium would not have been the impurity for this application since it would not freeze at liquid hydrogen temperatures.