Filling corrosive liquids: materials and safety

Before choosing materials it helps to know how the equipment fails. In corrosive liquids three typical mechanisms appear: pitting corrosion, triggered by chlorides on stainless steel; crevice corrosion, which concentrates in seals, threads and stagnant zones; and direct chemical attack on the elastomers of seals and gaskets.

How aggressive a product is depends on its pH, its oxidising power, the presence of halides (chlorides, fluorides) and the working temperature: the same material can resist a diluted acid when cold and fail with that same acid concentrated and hot.

AISI 316L stainless steel is the baseline for contact parts: its molybdenum content gives better resistance to chloride pitting than 304, which is kept for the frame. When stainless is not enough —for example with strong reducing acids or concentrated hypochlorite—, the contact parts are solved with PVC, titanium or other materials as required.

In seals and gaskets the choice matters as much as the metal: PTFE (Teflon) has the broadest range; Viton (FKM) resists acids and hydrocarbons well; EPDM works with polar products and bases, but not with oils. In addition, the pipework is joined with Orbital TIG welding and argon backing to avoid sensitised zones where corrosion would start.

The LA MAGC Series doses with an Endress+Hauser electromagnetic flowmeter. Measuring by the Faraday principle, it has no moving parts inside the fluid: the internal liner and the electrodes are selected to resist the product, which reduces wear and the risk of contamination.

The product flows through closed piping, with no open tank, which prevents aeration, evaporation and spills. The anti-drip system with a retractable tray contains splashes on every cycle, a critical point when handling acids or agrochemicals.

Cleaning is CIP/SIP with a self-draining system and an automatic programme: the circuit is washed and, where applicable, sterilised without dismantling any part. This way the operator does not have to open the machine or contact corrosive residues, and no pockets of stagnant product remain, which is exactly where crevice corrosion would begin.

Many corrosives are also flammable (solvents, certain agrochemicals). In those cases the machine is built in an ATEX (explosion-proof) version, designed to work in classified atmospheres, with grounding and components that do not create ignition sources. The product safety data sheet (SDS) is the reference document: it defines compatible materials, working temperature and precautions, and it is best to have it from the start of the project.