These are often caused by scaling, crystallization products, remaining process fluids, sulphur remains, process additives, amino-hydrochloride (in systems in which one uses ammonia for the neutralization of acids), inhibitors, colours for plastics, scales in secundary systems, waterside scaling, cokes, other carbon scales and degrading

Precommisioning-scaling
 
Welding and rolling skins
surface corrosion
odd iron parts (brush remains)
treatment liquids
parts of grinding wheels
grease- and oil remains
sand, dirt.

Coating

powder coating
kataforese polish
wet polish
teflon
paint

Atmosferisc pollution 
 
Dust remains: Sodium, calcium and magnesium salts like oxides, nitrates, carbonates, sulphates, and phosphates
Oil remains combined with dust remains

Scale
 
Ca and Mg- carbonates
Ca and Mg sulphate
Ca and Mg phosphate
Ca and Mg silicate
Ca and Mg oxides
Ca and Mg hydroxides
Ca and Mg fluorides 
 
Corrosion products

iron sulphates
iron fluorides
iron thiocyanates
iron cyanides
iron sulphides
iron chlorides
iron oxides
 
Organic polymers and rubbers

Polybutadieen-popcorn
Polybutadieen-styrene
Poly-etheenoxide
Poly-isoprene
Poly-ethylene
Poly-propene
Polymer
EPDM-rubber scales
Polystyrene Precommisioning scales
 
Hydro carbons 

Oil remains
Black Hydro carbon scales
carbon
Tar remains
Quench oils
Synthetic (compressor) oil 

Salts
 
Anorganic salts
Ammonium chlorides
Aluminium chlorides
Ammonium bisulfides 
 
Corrosion-products

iron oxides
magnetite
iron hydroxides 
 
Microbiological scales

bacteria
algae
weeds
shells, mussels
 
Fireside-scales 
 
Sodium vanadate
Vanadium trioxide
Sodium pyrosulphate
Pb-pyrosulphate
Silica
Iron oxides
Iron sulphate
Sodium chloride

Others

copper scales
Al-salts