DIN 50018 decides whether a car part survives the acid that condenses on it. Clean dew corrodes slowly. Dew that has dissolved sulphur dioxide is acid. Acid corrodes hard. The Kesternich test, which DIN 50018 sets out, doses a measured charge of sulphur dioxide into a warm, condensing chamber so the film that forms on the part is sulphurous acid, the corrosion of an industrial sky. The chamber has to make that acid dew form on schedule, survive it without corroding itself, and hold the toxic gas safely inside.
DIN 50018 is the German standard behind the Kesternich test, a corrosion test that condenses an acid dew onto a part. Plain cyclic condensation lays clean water on a surface and lets it corrode at the gentle rate clean water allows. The Kesternich test changes one thing. It adds sulphur dioxide to the chamber, so the water that condenses is no longer clean. The gas dissolves into the film and turns it to sulphurous acid, a far more aggressive electrolyte than water. A part that resists plain dew can corrode under the same dew once it carries acid. That difference, the gas and the acid it makes, is the entire subject of the test. The amount of gas is small. A measured charge, admitted once a cycle, is enough to turn every drop of condensation on the part acid for hours. The corrosion that follows is out of all proportion to the trace of gas that caused it, which is exactly the point. A little sulphur in the air of an industrial town does years of damage to a car. The test compresses that small, steady poison into a charge a chamber can dose and a part can meet in days.
The test takes its name from Wilhelm Kesternich, who set it out in 1951 to reproduce the damage that acid rain does to metal. The years after the war were the years of heavy coal smoke. The sulphur in that smoke fell back as acid on everything outdoors. Kesternich built a chamber test that put a small, controlled dose of sulphur dioxide into a warm, condensing atmosphere, so a part could meet in days the acid attack it would otherwise meet over years in an industrial town. The method outlived the smog that prompted it. A car still meets sulphur, in the exhaust of the traffic around it, in some fuels and roads, in the industrial air of the places it is built and driven. DIN 50018 carries the Kesternich method as a written standard, so a coating or a part can be proven against that acid in a way one laboratory can repeat for another.
The chemistry behind the test is what gives it its severity. Sulphur dioxide is a gas that dissolves readily in water. When it does, it forms sulphurous acid. A film of that acid on a metal surface is a strong electrolyte, far more conductive and far more reactive than a film of clean water. Clean dew corrodes slowly. Acid dew corrodes fast, working through the pores of a plating and under a paint film from any break in it. The corrosion continues as long as the film is wet. A fresh dose of gas in the next cycle renews it. This is why a part that passes a clean cyclic condensation test can fail the Kesternich. The water is the same. The gas dissolved in it is not.
The corrosion the acid leaves has its own look. On zinc and galvanised steel it is a white powdery bloom. On bare steel it is red rust, spreading from any cut or pore. On a painted part it is a creep of corrosion under the paint, starting at a scribe or a chip and lifting the film as it goes. The test draws out each of these in the part prone to it. The depth and spread of the attack after the cycles is the measure of how well the coating or the metal held. The reason the acid works so fast is electrochemical. A corroding metal sets up tiny cells across its surface, where one spot gives up metal and another takes the charge. A current runs between them through the film. Clean water carries that current poorly, so the cells run slow. Acid carries it readily, so the cells run hard. The metal dissolves at the active spot far quicker than clean dew would let it. The sulphur in the film does more than acidify it. Its compounds take part in the reaction, keeping it going where clean water would let it stall.
Zinc shows this plainly. In clean air a zinc surface builds a film of basic carbonate that slows further loss, the patina that lets a galvanised roof stand for decades. Sulphur dioxide attacks that patina and turns it into a soluble zinc sulphate. The sulphate washes off in the next wet phase and leaves bare zinc to corrode again. The metal never builds the layer that would save it. A sulphur atmosphere is far harder on zinc than clean country air, which is the damage the Kesternich exists to find.
Clean dew is water. Kesternich dew is acid.
The Kesternich test runs on a daily cycle, split into a wet half and a dry half. For the first eight hours the chamber is warm, around forty degrees, and saturated, so condensation forms on the part. The dose of sulphur dioxide makes that condensation acid. The part sits under its acid dew for those eight hours, the corrosion running at full speed. For the next sixteen hours the chamber is opened to the air and the part dries, the acid film evaporating and leaving its corrosion product behind. Then the cycle begins again, with a fresh dose of gas and a fresh condensation. A full test runs a set number of these twenty-four-hour cycles, more cycles for a harsher requirement. The count is what the standard fixes for a given part or coating.
How the dew itself forms is the subject of the clean cyclic condensation test. A reader who wants the physics of the dew point can find it there. What the Kesternich adds on top is the gas. The wet half is not merely damp; it is acid. The dry half is not merely a rest; it is the drying that concentrates the acid’s residue and readies the surface for the next dose. The alternation of acid wet and open dry is the stress. It is harder than either a constant acid soak or a clean condensation alone.
A car is not a laboratory coupon. The parts a car carries face sulphur in more places than one. The exhaust stream of the traffic around it carries oxides of sulphur. Some fuels and some roads add their own. The industrial air of a port, a tunnel or a factory district lays a sulphur film on anything parked in it. Under the body, where road spray and condensation already gather, that sulphur turns the damp acid. The parts that meet it are the brackets, the fasteners, the housings, the connectors and the coated panels that live in the wet, dirty underside of a car. DIN 50018 exists to prove the coatings and the metals of those parts against that acid before they fail on the road. A part qualified to the clean condensation test alone has been proven against water. The Kesternich proves it against the acid that water becomes near sulphur.
A car wears many finishes. Each corrodes in its own way. Zinc and zinc coatings, the workhorse protection of an underbody part, corrode to a white powder the test brings out quickly. The rate of that white rust shows how long the zinc will protect the steel beneath. Galvanised steel is judged on how long its zinc lasts before the red rust of the base steel breaks through. A painted or powder-coated panel is judged on whether acid can spread under the film from a scratch, a test of the bond between the coating and the metal. Aluminium and its coatings resist the acid better, without being immune, pitting where the protective oxide is broken. A part of mixed metals, a steel bracket with an aluminium fitting, corrodes fastest of all at the join, where the two metals form a galvanic cell in the acid. The Kesternich rates each of these on its own, which is why a maker runs it on the finished part, where a clean sample of one metal would miss the joins and the flaws.
A zinc or galvanised finish is often scored by how long it lasts. The number of cycles a coating survives before the base steel first shows red rust is a direct measure of the protection it gives. A finish that reaches red rust in a few cycles is weak; one that holds for many is strong. Two coatings can be ranked by that count alone, run side by side until each fails. The same logic reads a painted finish by the creep of corrosion out from a scribe after a fixed number of cycles, the smaller creep marking the better bond. These counts give a maker a number to specify against, so a drawing can call for a coating that survives a stated number of Kesternich cycles before red rust appears.
The gas is dosed, not flowed. The dose is what sets the severity. A measured volume of sulphur dioxide is admitted into the sealed chamber at the start of the wet phase. That fixed charge is what makes the dew acid for the cycle. A small dose makes a milder acid; a larger dose makes a harsher one. The standard names the dose for a given test. The variants of the test are distinguished by exactly that, the amount of gas put in each cycle. The chamber has to admit the dose accurately and seal it in, because a leak would both weaken the test and let a toxic gas escape. The repeatability the standard promises rests on the dose being the same, cycle after cycle and laboratory after laboratory. There is a discipline to the dosing. The gas is admitted as a measured volume, so its amount is fixed regardless of the chamber’s size. A larger vessel holds the same charge in a thinner concentration, so the standard names the dose against the chamber volume, letting two laboratories with different chambers run the same severity. A chamber that meters its gas loosely runs a test no one else can match.
The standard fixes the dose against a reference chamber of about three hundred litres and names each severity by the volume of gas admitted. The harsh level, written SFW 2.0 S, puts two litres of sulphur dioxide into that air, close to two-thirds of a percent by volume. A lighter requirement calls for a single litre, the SFW 1.0 S level near a third of a percent, or as little as two tenths of a litre for a mild exposure. The level names that proportion of gas, so a part proven to SFW 2.0 S has met a stronger sulphur atmosphere than one held to a lighter dose. The requirement fixes the level for a given part. A laboratory sets its charge to match it.
The condensation in a Kesternich chamber is made the simple way, with warmth and water. A reservoir of water sits in the bottom of the sealed vessel, heated so it evaporates into the closed air above. The air fills with vapour and the part, hung in that air, condenses the vapour onto its surface as the warmth and the geometry allow. The sulphur dioxide dosed into the same closed air dissolves into that condensing film, so what forms on the part is acid from the first drop. Hanging the part above the water, where a floor would let the dew pool and exhaust itself, lets the acid dew run off and renew. The chamber’s job in this is to keep the water warm, the air saturated and the gas held in, so every part hung inside is wetted by the same acid film for the same eight hours. Nothing about the condensation is forced. There is no spray, no pump, no fine control of where the dew lands. The warmth of the water and the cooling of the part decide it between them, the same way the night air and a cold panel form the dew on a car. The chamber’s only levers are the temperature of the water and the seal that keeps the air saturated. With those two it makes a dew as natural as the one the part will meet outdoors, only acid.
A chamber that fills itself with acid has to survive what it makes. The sulphurous and sulphuric acid that forms in a Kesternich run attacks the chamber as readily as the part. Ordinary stainless steel is not safe from it. So the vessel is lined or built from materials that hold against the acid, glass, certain plastics, or special coatings. Every seal, gasket and fitting that touches the atmosphere is chosen to resist it too. A chamber that corrodes its own walls fails twice over. It loses the materials of its construction into the test, where they can change the chemistry the part sees. It shortens its own life with every run. The lining is not a refinement on a Kesternich chamber. It is the difference between a chamber that runs the test and one the test destroys. The choice of lining shapes the entire installation. A glass-lined vessel resists the acid completely but is heavy and fragile. A plastic-lined one is lighter, with a limit on the temperature it can take. The fittings, the racks that hold the parts, even the thermometer well, all have to resist the acid they meet. A Kesternich chamber is in this sense a piece of chemical plant, built to contain a reaction and to hold a climate.
Sulphur dioxide is toxic. A chamber built to hold it carries a duty that a plain humidity chamber does not. The gas has to be sealed in through the wet phase, so none reaches the people around the chamber. When the chamber opens to ventilate, the air it releases has to be cleared of the gas, drawn through a scrubber that neutralises the sulphur before it leaves. The dosing line, the cylinder and the seals are all part of a system designed to keep a poison contained. None of this touches the part under test. It belongs to a Kesternich installation alongside the heater and the water. A laboratory that runs the test runs it inside a safety system built around the gas, because the same property that makes the test severe makes it dangerous to the people who run it. The scrubber is the heart of that safety. As the chamber vents at the end of the wet phase, its sulphur-laden air is pulled through a bed or a wash that binds the gas, so what reaches the room is clean. The dosing cylinder is kept and changed under its own precautions. A gas detector watches the air around the chamber for a leak. These measures cost space and care that a plain humidity chamber never demands. They are the price of running a test whose power is a poison.
A test is only fair if every part in it meets the same acid. A Kesternich chamber hung with many parts has to spread its dosed gas and its warm vapour evenly, so a part in one corner takes the same acid film as a part in another. If the gas pools at one end, the parts there meet a stronger acid than their neighbours. If a corner runs cool, the dew there is thin and the part under-tested. The reservoir, the air movement and the spacing of the parts are arranged so the dose disperses and the condensation forms uniformly across the load. A run that wets one part harder than the rest does not compare them fairly. The comparison of one coating against another is often the point of the test.
The corrosion settles on the weak point of a finish before the strong. On a plated part it begins at a pore or a thin spot, where the base metal lies closest to the surface. A bloom of white or red corrosion grows there. The sound plating around it still holds. On a painted part it opens at a scribe, a chip or an edge, anywhere the film is broken, and spreads under the paint from that break. On a part of two metals it concentrates at the join, where the more active metal gives itself up. The Kesternich draws out these flaws because its acid corrodes what clean water leaves alone. A coating that looks sound can carry a pinhole that plain dew would never reach. The acid dew of the test reaches it and opens it into a visible failure on the bench.
A Kesternich run that falls short of its conditions hides the failures it exists to find. If the dose of gas is too small, the dew is too weak an acid. A marginal coating passes that an industrial winter would corrode. If the chamber leaks its gas, the acid thins as the cycle runs. The later hours test less than the standard intends. If the warmth or the saturation fall away, the dew is thin and there is little water for the acid to form in. Each of these softens the test without obviously failing it. Each lets a part through that will later fail in the field. The result of a Kesternich test is only as honest as the acid the chamber held, dose by dose and cycle by cycle.
The part is read for what the acid took. The first read is by eye, for the bloom of corrosion, the rust, the blistering or the creep of a coating, rated against a scale the standard sets. A coated part is judged on how far the corrosion has spread from a deliberate scribe, the measure of how well the coating resists the acid creeping under it. A metal part can be weighed before and after, the loss of mass telling how much metal the acid dissolved. Where a number leaves a doubt, the part is sectioned to see how deep the attack ran. The chamber stands behind every one of these reads. A corrosion grown under a clean, full-strength acid dew is the part’s own result. A corrosion read after a soft or leaking run means little, since a weak finish cannot be told from a weak test. The reading is often comparative. A maker rarely asks only whether one coating passes. It asks which of several holds best, running them side by side through the same cycles. A reference panel of known behaviour goes in with them, so the run can be judged against a result the laboratory trusts. The Kesternich’s value lies in ranking finishes, beyond a simple pass or fail. That ranking is sound only if every panel met the same acid.
The Kesternich is one of a family of corrosion tests, each tuned to a different attacker. A clean cyclic condensation test lays plain dew, for parts that meet honest damp without much chemistry. A salt spray test throws a salt mist, for the chloride attack of a coastal or a salted road. The Kesternich dews an acid, for the sulphur of an industrial or a traffic-laden air. The three are not interchangeable. Salt corrodes by one path and sulphur by another. A coating strong against one can be weak against the other. A maker chooses the Kesternich when the threat is sulphur and the acid it makes, runs a salt spray when the threat is salt, and runs a clean condensation when the threat is only the water itself.
What the Kesternich asks of a chamber is to make a controlled acid and survive it. The chamber has to hold a warm, saturated wet phase for its eight hours. It has to admit an exact dose of sulphur dioxide and seal it in, so the dew becomes the acid the standard names. It has to ventilate and dry the part for its sixteen hours, then dose and condense again, cycle after cycle. It has to do this without corroding its own lined walls, and without letting a breath of the poison reach the people around it. A chamber that delivers all of that lets a maker prove a coating or a part against the acid of an industrial sky. The result is a promise that the finish will hold where sulphur and damp meet. The chamber is what makes the promise honest, one dosed cycle at a time.
The Kesternich test is a corrosion test that condenses an acid dew onto a part by adding sulphur dioxide to a warm, humid chamber. DIN 50018 is the German standard that sets it out. Wilhelm Kesternich devised it in 1951 to reproduce acid-rain damage. It is used to prove coatings, platings and base metals against the sulphur-laden, acid atmospheres of industrial and traffic environments.
Because clean condensed water corrodes slowly. The same water with sulphur dioxide dissolved in it becomes sulphurous acid, a far more aggressive electrolyte. The acid attacks zinc, steel, platings and paint far faster than clean dew. It reaches the pores and flaws in a finish that plain water would never touch. The gas is what makes the test reproduce a sulphur-bearing atmosphere rather than ordinary damp.
It is a 24-hour cycle in two halves. For about 8 hours the chamber is warm, near 40 C, and saturated, with a dose of sulphur dioxide, so acid dew forms on the part. For the next 16 hours the chamber ventilates and the part dries, leaving the corrosion behind. The cycle repeats for the number of times the requirement sets, with a fresh dose of gas each time.
Because the acid it makes attacks the chamber as readily as the part. Ordinary stainless steel is not safe from sulphurous and sulphuric acid, so a Kesternich vessel is lined or built from glass, certain plastics or special coatings, with seals and fittings chosen to resist the acid. A chamber that corrodes its own walls both shortens its life and changes the chemistry the part sees.
Each test reproduces a different threat. Plain cyclic condensation lays clean dew for ordinary damp. Salt spray throws a chloride mist for coastal and road-salt corrosion. The Kesternich dews an acid for the sulphur of industrial and traffic air. A coating strong against salt can be weak against acid, so the test is chosen to match the threat the part will meet.
Envsin reliability and environmental test chambers for cyclic condensation and Kesternich sulphur-dioxide corrosion testing.