Water in its natural form is not pure but contains a variety of dissolved substances depending on its origin (ground water or surface water). Water can contain these substances as minerals, free acids or gases, its chemical behaviour being influenced to a lesser or greater degree according to the quantity and type of the substances. Untreated water often contains also undissolved substances of an inorganic and organic nature as suspended or settling matter.

Natural water

Drinking water must be colourless, odourless, clear and largely free of germs. It is governed by the regulations of the Food Act. The only way to determine its corrosive behaviour is by analysis. Mineral water is water obtained from natural or man-made springs; it has a higher content of minerals, free carbon dioxide and rare constituents of pharmacological significance such as bromine, iodine, sulphur, radium, iron and others. Mineral water is often highly corrosive, especially at temperatures above the ambient temperature. Brine is natural, saliferous water with a mass content of minerals (mainly NaCI) of at least 1.5%; it is highly corrosive.

Thermal waters is water whose temperature is naturally higher than 20 °C; its mineral content is irrelevant. The only way to establish its corrosive action is by analysis.

Pit water often contains considerable proportions of metallic salts and metallic oxides, sometimes subject to great fluctuations. Pit water is mostly highly corrosive.

Sea water is highly corrosive due to its oxygen content and its mineral-induced conductivity; its chloride content prevents formation of effective protective coatings. Following mineral contents (mainly NaCI and MgCI) are found: 3.3 – 3.75% in the oceans, 3.2% in the North Sea, 0.6 – 1.9% in the Baltic Sea, 3.8 % in the Mediterranean Sea and 21.7 % in the Dead Sea.

Brackish water is a mixture of fresh water and sea water that fluctuates greatly depending on the tides and water level; it is mostly highly corrosive.

Sewage water is water polluted by domestic, commercial and industrial usage (waste water), and precipitation running off developed land (rain water). On account of the greatly fluctuating composition of sewage water, the only way to determine its material-attacking properties is by analysis.

Treated water

A distinction is drawn between:

Partially demineralized water, i. e. water with an electrolyte content corresponding to a conductivity of ³ 0.2 mS/cm, measured behind high-acid sampling-analysis cation exchangers.

Fully demineralized water (deionized water). Full demineralization, i. e. the removal of all ions from the water, is performed using ion exchangers and gravel bed filters, with expulsion of free carbon dioxide in a follow-up operation. Oxygen is usually bound by hydrazine or sodium sulfite. Boiler feed water (feed water treatment) is a special category of fully demineralized water.