Chemical Inhibition Consideration in Commissioning and Decommissioning Clean

Materials Consideration

In the system, there are normally multiple materials in use.  Some principles include:

• Austenistic and duplex stainless steel  should be with great care to use hydrochloric acid or water with high chloride content;
• In the system using mixture of ferrous and nonferrous metals, the deposition of copper on ferrous materials should be avoided;
• Cast iron suffers more corrosion attack at elevated temperature while using hydrochloric acid cleaning;
• Dezincification in strong acid/ caustic with high zinc brasses (≥15% w/w Zn);
• Denickelification of Monel alloy in hydrofluoric and other acids.

Temperature Consideration

• Normal metallic materials, up to 75^oC in hydrochloric acid;
• Not exceed sensitisation temperature limit for austenistic stainless steel. The ferric ion content in the acid should be taken into consideration;
• Cast iron, up to 50^oC in hydrochloric acid;
• Thermal cycle and condensation of acidic fume, which cause excessive corrosion attack at headspace;
• Particular attention to confined headspace for explosive hazards.

Fluid Corrosivity Consideration

• Spent acid should not be recycled: it may cause cross contamination and unexpected corrosion issue;
• Cupric ion in the acid is detrimental to the ferrous materials;
• Ferric ion in the acid is corrosive to both ferrous and nonferrous materials, should be limited to lower threshold (≤0.4%w/w);
• Proper corrosion inhibitors are required for the cleaning process.

Cleaning Practices

• Only fresh acid solution is used until the cleaning completed as in most of commissioning and decommissioning cleaning process;
• Cleaning solution is replenished during cleaning cycle as in acid pickling process and cleaning process for electroplating and electroless plating.