Corrosion Glossary - S

Sacrificial coating: a coating that provides corrosion protection wherein the coating material corrodes in preference to the substrate, thereby protecting the latter from corrosion.
Safety shut-off valve: a manually opened, electrically latched, electrically operated safety shut-off valve designed to automatically shut off fuel when de-energized.
Safety valve: a spring loaded valve that automatically opens when pressure attains the valve setting. used to prevent excessive pressure from building up in a boiler.
Sagging (or running, curtaining): the unsightly gravity driven flow that usually occurs on vertical surfaces. This is due to too much flow, often related to application technique or environment.
Saline water: water containing an excessive amount of dissolved salts, usually over 5,000 mg/l.
Salinity: the total proportion of salts in seawater, often estimated empirically as chlorinity x 1.80655, also expressed in parts per thousand, i.e. o/oo
Salt: in chemistry, the term is applied to a class of chemical compounds which can be formed by the neutralization of an acid with a with a base; the common name for the specific chemical compound sodium chloride used in the regeneration of ion exchange water softeners.
Salt fog: ASTM B-117 test procedure that attempts to simulate the corrosive environment caused by road salt and marine spray.
Salt splitting: the process in which neutral salts in water are converted to their corresponding acids or bases by ion exchange resins containing strongly acidic or strongly basic functional groups.
Sampling: the removal of a portion of a material for examination or analysis.
Sand blasting (also grit blasting): the process of surface cleaning and roughening that provides a mechanical "tooth" to aid coating adhesion. Media include aluminum oxide, even crushed walnut shells. The medium must be chosen to match the substrate and the foreign material on the substrate to be removed.
Saturated air: air which contains the maximum amount of water vapor that it can hold at its temperature and pressure.
Saturated steam: steam at the temperature and pressure at which evaporation occurs.
Saturated temperature: the temperature at which evaporation occurs at a particular pressure.
Scale: a deposit of mineral solids on the interior surfaces of water lines and containers, often formed when water containing the carbonates or bicarbonates of calcium and magnesium is heated. Oxide of iron that forms on the surface of steel after heating.
Scanning electron microscope (SEM): an electron microscope in which the image is formed by a beam synchronized with an electron probe scanning the object. The intensity of the image forming beam is proportional to the scattering or secondary emission of the specimen where the probe strikes it
Scoring: a severe form of wear characterized by the formation of extensive grooves and scratches in the direction of sliding.
Sealant, sealer: a preparation of resin or wax type materials for sealing the porosity in coatings.
Sealing: a process which, by absorption of a sealer into thermal spray coatings, seals porosity and increases resistance to corrosion of the underlying substrate material.
Season cracking: See stress-corrosion cracking (SCC).
Secondary treatment: treatment of boiler feed water or internal treatment of boiler-water after primary treatment.
Sediment: matter in water which can be removed from suspension by gravity or mechanical means.
Sedimentation: the process in which solid suspended particles settle out of water, usually when the water has little or no movement. Also called "settling".
Seeds: (paint) small granule-like defects which occur randomly over a coating surface marring the appearance. Seeds can result from undispersed or flocculated pigment, dirt, resin gel particles, precipitated resin, and pigment due to solvent shock.
Segregation: the tendency of refuse of varying compositions to deposit selectively in difference parts of the unit.
Self-bonding coatings: a name given to thermal spray coatings that are capable of bonding to clean smooth surfaces. Bond and "one-step" coatings are normally in this group. These are paticularly important where grit blasting or surface roughening processes must be omitted.
Semi-finished steel: steel shapes, for example blooms, billets or slabs, that later are rolled into finished products such as beams, bars or sheet.
Semipermeable membrane: typically a thin, organic film which allows the passage of some ions or materials while preventing the passage of others. Some membranes will only allow the passage of cations.
Septic: a condition existing during the digestion of organic matter, such as in sewage, by anaerobic bacteria in the absence of air. A common process for the treatment of household sewage in septic tanks, and in municipal sewage treatment in specially designed digester.
Sequestering agent: A chemical that causes sequestration when fed into water. For example, polyphosphates are sequestering agents which sequester hardness and prevent reactions with soap. If the ions involved are metal ions, sequestering agents may also be chelating agents.
Sequestering agent: a chemical compound sometimes fed into water to tie up undesirable ions, keep them in solution, and eliminate or reduce the normal effects of the ions. For example, polyphosphate can sequester hardness and prevent reactions with soap.
Service run: That portion of the operating cycle of a water conditioning unit in which treated water is being delivered, as opposed to the period when the unit is being backwashed, recharged or regenerated.
Service water: general purpose water which may or may not have been treated for a special purpose.
Shear: that type of force that causes or tends to cause two contiguous parts of the same body to slide relative to each other in a direction parallel to their plane of contact.
Shear strength: the stress required to produce fracture in the plane of cross section, the conditions of loading being such that the directions of force and of resistance are parallel and opposite although their paths are offset a specified minimum amount. The maximum load divided by the original cross-sectional area of a section separated by shear.
Sheet: wide, flat-rolled steel. It is generally accepted that steel less than 3 mm thick is sheet and more than 3 mm (1/8 inch) thick is plate (See Plate).
Shelf life: (paint) the length of time any unopened container can be stored at the supplier recommended storage temperature and still retain the properties in both the unmixed and mixed states as required by the specification or advertised in the product data sheets.
Shell: the cylindrical portion of a pressure vessel.
Shielded: the separation of metallic parts by an electrical nonconductor; insulated by other than an air gap.
Shrinkage: a decrease in dimensions of a coating during processing.
Shrinkage stress: the residual stress in a coating caused by shrinkage during processing.
Shot peening: the bombardment of a component surface with steel or ceramic shot. Produces a residual compressive stress in the surface and improves fatigue and stress corrosion performance.
Shroud: a gaseous and/or mechanical or physical barrier placed around the spraying process designed to reduce the ingress of air into the system and so reduce oxidation of the of the materials being sprayed.
Sidedraft booth: a paint spray booth in which the air movement is from the front to the back of the booth.
Sieve classification: that portion of a powder sample which passes through a standard sieve of specified number and is retained by some finer sieve of specified number.
Sigma phase: a hard, brittle, nonmagnetic intermediate phase with a tetragonal crystal structure, containing 30 atoms per unit cell occurring in many binary and ternary alloys of the transition elements.
Sigma-phase embrittlement: embrittlement of iron-chromium alloys (most notably austenitic stainless steels) caused by precipitation at grain boundaries of the hard, brittle intermetallic sigma phase during long periods of exposure to temperatures between approximately 560 and 980ºC. Sigma-phase embrittlement results in severe loss in toughness and ductility, and can make the embrittled material susceptible to intergranular corrosion.
Silking: this is defined as fine parallel irregularities in a paint film that give the appearance of silk. This defect usually is a special case of floating and flocculation in coating finishes.
Silver plating: the electrodeposition of silver for electrical, decorative or anti-fretting properties.
Silica gel or siliceous gel: a synthetic hydrated sodium aluminosilicate with ion exchange properties, once widely used in ion exchange water softeners.
Silicone: a resin used in the binders of coatings. Also used as an additive to provide specific properties, e.g., defoamer. Paints containing silicone are very slick and resist dirt and bacterial growth, and are stable in high heat.
Skinning: the formation of a thin, tough film on the surface of a liquid paint. Commonly caused by a chemical reaction to moisture in the air.
Slab: a semi-finished, hot-rolled section of flat-rolled steel, prepared for rolling down to plate or sheet. It is generally more than 4 cm thick and more than twice as wide as it is thick.
Slag: a) The non-metallic material forming a molten layer on top of the molten steel in a steel furnace. It is made by charging suitable materials and plays an important role in the refining of the steel. b) Loosely applied to any waste material drawn off in molten form.
Slip: plastic deformation by the irreversible shear displacement (translation) of one part of a crystal relative to another in a definite crystallographic direction and usually on a specific crystallographic plane.
Slow strain rate technique: an experimental technique for evaluating susceptibility to stress-corrosion cracking. It involves pulling the specimen to failure in uniaxial tension at a controlled slow strain rate while the specimen is in the test environment and examining the specimen for evidence of stress-corrosion cracking.
Sludge: the semi-fluid solid matter collected at the bottom of a system tank or watercourse, as a result of the sedimentation or settling of suspended solids or precipitates.
Slug: a large "dose" of chemical treatment applied internally to a steam boiler intermittently. also used sometimes instead of "priming" to denote a discharge of water out through a boiler steam outlet in relatively large intermittent amounts.
Slushing compound: an obsolete term describing oil or grease coatings used to provide temporary protection against atmospheric corrosion.
Smelt: molten slag; in the pulp and paper industry, the cooking chemicals tapped from the recovery boiler as molten material and dissolved in the smelt tank as green liquor.
Smelter: facility is used to extract metal concentrates found inside mined ore. The ore will often contain more than one kind of metal concentrate and this facility also separates them.
Smoke: small gas borne particles of carbon or soot, less than 1 micron in size, resulting from incomplete combustion of carbonaceous materials and of sufficient number to be observable.
S-N diagram: a plot showing the relationship of stress, s, and the number of cycles, n, before fracture in fatigue testing.
Soda ash: the common name for sodium carbonate, Na2CO3, a chemical compound used as an alkalinity builder in some soap and detergent formulations to neutralize acid water, and in the lime-soda water treatment process.
Sodium chloride: the chemical name for common salt.
Sodium cycle: the cation exchange process in which sodium on the ion exchange resin is exchanged for hardness and other ions in water. Sodium chloride is the common regenerant used in this process.
Soft water: water which contains little or no calcium or magnesium salts, or water from which scale forming impurities have been removed or reduced.
Softening: the act of reducing scale forming calcium and magnesium impurities from water.
Solar energy: energy derived from our sun. Strictly speaking, direct solar radiation, hydroelectric, wind, and even biomass and fossil energy are all forms of solar energy since none would be present without the existence of the sun. However, for this glossary, only direct solar radiation is denoted by this term.
Solid solution: a single, solid, homogeneous crystalline phase containing two or more chemical species.
Solute: the component of either a liquid or solid solution that is present to a lesser or minor extent: the component that is dissolved in the solution.
Solute: the substance which is dissolved in and by a solvent. Dissolved solids, such as the minerals found in water, are solutes.
Solution: a homogeneous dispersion of two or more kinds of molecular or ionic species. Solutions may be composed of any combination of liquids, solids, or gases, but they always consist of a single phase.
Solution feeder: a device, such as a power driven pump or an eductor system, designed to feed a solution of a water treatment chemical into the water system, usually in proportion to flow.
Solution heat treatment: heating an alloy to a suitable temperature, holding at that temperature long enough to cause one or more constituents to enter into solid solution, and then cooling rapidly enough to hold these constituents in solution.
Solvent: the component of either a liquid or solid solution that is present to a greater or major extent; the component that dissolves the solute.
Solvent degreasing: the removal of oil, grease, and other soluble contaminants from the surface of the workpiece by immersion in suitable cleaners.
Solvent popping: the formation of defects by the violent evolution of trapped solvent or carbon dioxide that occurs after the coating has begun to gel during its curing cycle. Commonly caused by a combination of the following, high film builds, high temperature and relative humidity, high airflow, quick curing, slow evaporating solvents, and certain types of spray equipment.
Soot: unburned particles of carbon derived from hydrocarbons.
Sorption behavior: the ability of a hygroscopic product to absorb or release water vapor from or into the air until a state of equilibrium is reached.
Sour gas: a gaseous environment containing hydrogen sulfide and carbon dioxide in hydrocarbon reservoirs. prolonged exposure to sour gas can lead to hydrogen damage, sulfide-stress cracking, and/or stress-corrosion cracking in ferrous alloys.
Sour water: waste waters containing fetid materials, usually sulfur compounds.
Spalling: the breaking off of the surface of refractory material as a result of internal stresses or the lifting or detachment of a coating from the substrate.
Specialty steel: steels such as electrical, alloy or stainless steels. These generally are produced in smaller volumes to meet the specific needs of customers.
Specific conductance: the measure of the electrical conductance of water or a water solution at a specific temperature, usually 25oC.
Specific gravity: the ratio of the weight of a specific volume of a substance compared to the weight of the same volume of pure water at 4oC.
Specific humidity: the weight of water vapor in a gas water-vapor mixture per unit weight of dry gas.
Specular gloss: mirror-like finish (usually 60 degrees on a 60-degree meter).
Spheroidite: an aggregate of iron or alloy carbides of essentially spherical shape dispersed throughout a matrix of ferrite.
Splat: a single thin flattened sprayed particle.
Splat cooling: extremely rapid, high rate of cooling, the effects of which can be observed in thermal spraying deposits leading to the formation of metastable phases and an amorphous microstructure.
Spray angle: the angle included between the sides of the cone formed by liquid discharged from mechanical, rotary atomizers and by some forms of steam or air atomizers.
Spray-fused coatings: a process in which the coating material is deposited by flame spraying and then fused into the substrate by the addition of further heat. This can be applied by flame, induction heating or by laser.
Spray nozzle: a nozzle from which a liquid fuel is discharged in the form of a spray.
Sputtering: a coating process whereby thermally emitted electrons collide with inert gas atoms, which accelerate toward and impact a negatively charged electrode that is a target of the coating material. The impacting ions dislodge atoms of the target material, which are in turn projected to and deposited on the substrate to form the coating.
Stack: a vertical conduit, which due to the difference in density between internal and external gases, creates a draft at its base.
Stack draft: the magnitude of the draft measured at the inlet to the stack.
Stagnation: the condition of being free from movement or lacking circulation.
Standard electrode potential: the reversible potential for an electrode process when all products and reactions are at unit activity on a scale in which the potential for the standard hydrogen half-cell is zero.
Static pressure: the measure of potential energy of a fluid.
Static system: a system or process in which the reactants are not flowing or moving, i.e the opposite of dynamic system.
Steam: the vapor phase of water, unmixed with other gases.
Steam generating unit: a unit to which water, fuel, and air are supplied and in which steam is generated. it consists of a boiler furnace, and fuel burning equipment, and may include as component parts water walls, superheater, reheater, economizer, air heater, or any combination thereof.
Steam generator: machine using heat in a power plant to produce steam to turn turbines.
Steam separator: a device for removing the entrained water from steam.
Steam tempering: the production of a stable oxide on steel parts by treatment in steam at about 300oC. Improves corrosion performance and reduces friction.
Strain: the unit of change in the size or shape of a body due to force.
Strain hardening: an increase in hardness and strength caused by plastic deformation at temperatures below the recrystallization range.
Strain rate: the time rate of straining for the usual tensile test.
Strainer: a device, such as a filter, to retain solid particles allowing a liquid to pass.
Stratification: non-homogeneity existing transversely in a gas stream.
Stray current: current flowing through paths other than the intended circuit.
Stray current corrosion: the corrosion caused by electric current from a source external to the intended electrical circuit, for example, extraneous current in the earth.
Stress: the intensity of the internally distributed forces or components of forces that resist a change in the volume or shape of a material that is or has been subjected to external forces. Stress is expressed in force per unit area and is calculated on the basis of the original dimensions of the cross section of the specimen.
Stress concentration factor (K_t): a multiplying factor for applied stress that allows for the presence of a structural discontinuity such as a notch or hole; Kt equals the ratio of the greatest stress in the region of the discontinuity to the nominal stress for the entire section.
Stress-corrosion cracking: a cracking process that requires the simultaneous action of a corrodent and sustained tensile stress. (This excludes corrosion-reduced sections which fail by fast fracture. It also excludes intercrystalline or transcrystalline corrosion which can disintegrate an alloy without either applied or residual stress.
Stress-intensity factor: a scaling factor, usually denoted by the symbol K, used in linear-elastic fracture mechanics to describe the intensification of applied stress at the tip of a crack of known size and shape. At the onset of rapid crack propagation in any structure containing, a crack, the factor is called the critical stress-intensity factor, or the fracture toughness.
Stress raisers: changes in contour or discontinuities in structure that cause local increases in stress.
Stress-relief cracking: also called postweld heat treatment cracking, stress-relief cracking occurs when susceptible alloys are subjected to thermal stress relief after welding to reduce residual stresses and improve toughness. Stress-relief cracking occurs only in metals that can precipitation-harden during such elevated-temperature exposure; it usually occurs at stress raisers, is intergranular in nature, and is generally observed in the coarse-grained region of the weld heat-affected zone.
Stress relieving: heat treatment carried out in steel to reduce internal stresses.
Striation: a fatigue fracture feature, often observed in electron micrographs, that indicates the position of the crack front after each succeeding cycle of stress. The distance between striations indicates the advance of the crack front across that crystal during one stress cycle, and a line normal to the striation indicates the direction of local crack propagation.
Strip: steel rolled out into long, thin, flat strips. Steel up to about 60 cm wide is strip or narrow strip and wide strip above 60 cm.
Stud: a projecting pin serving as a support or means of attachment.
Substrate: the parent or base material to which a coating is applied.
Subsurface corrosion: See internal oxidation.
Sulfate-reducing bacteria (SRB): A group of bacteria which are capable of reducing sulfates in water to hydrogen sulfide gas, thus producing obnoxious tastes and odors. These bacteria have no sanitary significance, and are classed as nuisance organisms.
Sulfidation: the reaction of a metal or alloy with a sulfur-containing species to produce a sulfur compound that forms on or beneath the surface of the metal or alloy.
Sulfonic acid: A specific acidic group (SO₃H^-) which gives certain cation exchange resins their ion exchange capability.
Superchlorination: the addition of excess amounts of chlorine to a water supply to speed chemical reactions or insure disinfection with short contact time. The chlorine residual following superchlorination is high enough to be unpalatable, and thus dechlorination is commonly employed before the water is used.
Superheated steam: steam with its temperature raised above that of saturation. the temperature in excess of its saturation temperature is referred to as superheat.
Supernatant: the clear liquid lying above a sediment or precipitate.
Surface active agent: the material in a soap or detergent formulation which promotes the penetration of the fabric by water, the loosening of the soil from surfaces, and the suspension of many soils; the actual cleaning agent in soap and detergent formulations.
Surface blowoff: removal of water, foam, etc. from the surface at the water level in a boiler. the equipment for such removal.
Surface preparation: cleaning and roughening the surface to be sprayed, usually by grit blasting. This is to increase the adhesion of the coating to the substrate.
Surfacer: an easy sanding paint used to fill surface irregularities.
Surface resistivity: the resistance of a material between two opposite sides of a unit square of its surface.
Surface tension: the result of attraction between molecules of a liquid which causes the surface of the liquid to act as a thin elastic film under tension. Surface tension causes water to form spherical drops, and to reduce penetration into fabrics. Soaps, detergents and wetting agents reduce surface tension and increase penetration by water.
Surface topography: the geometrical detail of a surface, relating particularly to microscopic variations in height.
Surface water: water which is open to the atmosphere and subject to surface runoff; generally, lakes, streams, rivers, reservoirs.
Surfacing: the application of a coating or cladding to a surface to impart a change in its surface behavior.
Surfactant: a surface-active agent; usually an organic compound whose molecules contain a hydrophilic group at one end and a lipophilic group at the other.
Surfactant: a contraction of the term "surface-active agent".
Surge: the sudden displacement or movement of water in a closed vessel or drum.
Suspended solids: solid particles in water which are not in solution.
Sustainable development: a term used to connote ways humanity must learn to use and develop its resources in order to sustain a high quality of life on the planet Earth for periods well into the future.
Synthetic detergent: a synthetic cleaning agent, such as linear alkyl sulfonate and alkyl benzene sulfonate. Synthetic detergents react with water hardness, but the products are soluble.
System international (SI): The SI system of measurement, otherwise known as the metric system, is used in most countries around the world. It is based on factors of ten, and is convenient to use with scientific calculations and numbers that are very small or very large.

Link to glossary of corrosion and materials maintenance terms