Open tanks or containers, with or without agitators, with a total internal (geometric) volume greater than 0.5 m³ (500 litres), where all surfaces that come in direct contact with the chemical being processed or contained are made from any of the following materials— a alloys with more than 25 % nickel and 20 % chromium by weight; b fluoropolymers; c glass (including vitrified or enamelled coatings or glass lining); d nickel or alloys with more than 40 % nickel by weight; e tantalum or tantalum alloys; f titanium or titanium alloys; g zirconium or zirconium alloys; h niobium (columbium) or niobium alloys; i stainless steel; j wood; or k rubber, including all kinds of natural and synthetic rubbers.