Rust Prevention for Tool Steel in Storage and Transit
Tool steel usually rusts before it is put to work, not after. A bar that leaves the mill with a clean ground surface can arrive at a customer with a bloom of surface rust after a month in a coastal warehouse or a sea container, and the cost lands on the supplier. This page covers what the atmosphere actually does to a steel surface, the two pollutants that decide how bad a location is, how a location is classified, and the measures that keep a delivery in the condition it was despatched in.
Rust needs a wet surface, and the atmosphere decides how long it stays wet
The controlling variable in atmospheric corrosion is not temperature or humidity on their own. The source treats them as a pair, the temperature and humidity complex, and then adds the parameter that actually drives the corrosion, which is the time of wetness. That is the period during which moisture exists on the metal surface and corrosion can proceed. The moisture layer that matters is not always visible rain. It can be generated by rain, fog, snow, dew condensation, or by capillary condensation in a deposit or a gap, and the last of those is the one that quietly keeps a surface wet for weeks inside a sealed package.
The threshold the source uses comes from the ISO 9223 calculation. Time of wetness is counted using 80% as the critical relative humidity for temperatures at or above 0 C (32 F), because that is where condensation starts on a metal surface. Above 90% relative humidity and below 25 C (77 F) the dewpoint is reached and the moisture on the surface becomes visible, in a layer thicker than the film formed by first condensation. There is a useful surprise in the same paragraph. The thinner layer is a poor barrier to oxygen diffusion and becomes practically saturated in dissolved oxygen, so the corrosion rate of the metal is actually faster under the thin condensation film than under the thicker moisture layer that forms at higher relative humidity. Drying a surface a little is not the same as drying it properly.
| Condition | Threshold the source gives | What it means for bare steel |
|---|---|---|
| Time of wetness counting | 80% relative humidity at temperatures at or above 0 C (32 F) | Above that level the surface counts as wet in the ISO 9223 time of wetness calculation, because condensation starts there |
| Dewpoint and visible moisture | Above 90% relative humidity and below 25 C (77 F) | The dewpoint is reached, the moisture on the surface becomes visible, and the wet layer is thicker than the film from first condensation |
| First condensation film | The thinnest layer, formed first | It is a minor barrier to oxygen diffusion and is practically saturated in dissolved oxygen, so corrosion is faster under it than under a thicker layer at higher humidity |
| Rain | Any amount | It supplies electrolyte and it also dilutes and washes off the pollutants already deposited on the surface |
| Dust and aerosol particles | Any deposit | Most aerosol particles absorb water, which lengthens the time of wetness and lengthens the corrosion process with it |
The wetness thresholds the source uses. The counterintuitive entry is the third one, where the thinnest moisture layer turns out to be the most aggressive because it stays saturated in dissolved oxygen.
That set of numbers is what a warehouse decision turns on. A store that holds at 85% relative humidity for most of the year is a store where bare steel is wet by the definition the standard uses, even if no water is visible on the racks. A dehumidified store held below the critical value is not, and the difference in what happens to the stock is the difference between the two regimes rather than a gradual improvement.
The two pollutants that decide how bad a location is
The source names two pollutants as the ones that accelerate the atmospheric corrosion rate by several orders of magnitude, and both are relevant to a steel supplier. Chlorides arrive as airborne salinity in aerosols from salt spray and salt fog near a coast, and from plants producing hydrogen chloride or sodium hypochlorite. The chloride ion works directly against the mechanism that protects a stainless surface, penetrating and destroying the passive oxide and hydroxide layer, and it is one of the principal agents behind pitting attack. Sulfur dioxide is the urban and industrial pollutant, and it works indirectly. In the presence of oxygen it converts to sulfuric acid inside the condensed moisture layer on the metal, the moisture pH often falls below 4.5, and the corrosion rate rises sharply. Metals that are relatively resistant in a neutral atmosphere corrode rapidly in that acid environment.
| Pollutant | Where it comes from | How it attacks steel | How it is classified |
|---|---|---|---|
| Chlorides, airborne salinity | Aerosols from salt spray and salt fog near the seashore, and from plants producing hydrogen chloride or sodium hypochlorite | The chloride ion penetrates and destroys the protective passive oxide and hydroxide layer, and it is one of the principal agents behind pitting attack | Annual average deposition rate in mg per square metre per day under ISO 9223, sampled with the wet candle apparatus of ISO 9225 |
| Sulfur dioxide | Urban and industrial atmospheres, largely from burning fossil fuel | In the presence of oxygen it converts to sulfuric acid inside the condensed moisture layer, and the moisture pH often falls below 4.5, which accelerates the corrosion rate strongly | Annual average deposition rate in mg per square metre per day under ISO 9223, sampled with the sulfation plate of ISO 9225 |
| Other gases and organic acids | Carbon dioxide, nitrogen dioxide, ozone, ammonia, hydrogen sulfide and hydrogen chloride, plus formic and acetic acid | Once dissolved in the moisture layer they add ions that change the mechanism and raise the corrosion current from the electrochemical cells | Not part of the two pollutant classification, but they add to the measured rate |
The pollutants the source identifies as the principal accelerants of atmospheric corrosion, with the classification route for each. Both are classified the same way, by annual average deposition rate.
The classification route is the same for both, and it is worth knowing because it is the number a customer or a logistics decision can actually be attached to. Under ISO 9223 the atmospheric corrosivity is classified from the annual average deposition rate of chlorides and of sulfur dioxide, expressed in milligrams per square metre per day, and the recommended sampling methods are given in ISO 9225 with the wet candle apparatus for chloride and the sulfation plate for sulfur dioxide. A site near the coast and a site in an industrial valley will land in different categories on those two numbers alone, and the same crate of tool steel faces a different risk at each one.
What the steel can and cannot do about it
There is a thermodynamic frame for the whole subject, and the source applies it to iron because iron is the main constituent of carbon and alloy steels. A Pourbaix diagram for iron in water at 25 C shows three possibilities as potential and pH move, a corrosion region where the metal is active and dissolving, a passivity region where a protective oxide holds the surface, and an immunity region where the metal itself is stable. The practical reading for a tool steel order is that the three regions are set by the chemistry of the steel as well as by the environment. A stainless grade earns its place in the passive region because of its chromium. A carbon or low alloy tool steel has no such region to occupy in a normal atmosphere, so there is no surface state to aim for and protection has to come from outside.
That is why the two product groups need different answers to the same question. For a stainless grade in a coastal store, the question is whether the passive film survives the chloride level, and the answer is partly the grade and partly the finishing. For a carbon or low alloy tool steel, the question is only how long the barrier lasts, because a barrier is all there is. The chemistry that decides which group a grade falls into is on the stainless steel corrosion resistance chart and the tool steel composition chart.
What keeps a delivery in condition
The measures that work follow from the mechanism. Corrosion needs a wet surface, so the first lever is the water and the second is the time it stays there. Where a part is going into a closed package, the answer the source gives is a vapor phase corrosion inhibitor, a volatile compound with a moderately high vapor pressure that reaches the metal through the vapor phase instead of being applied to it, and that is effective in enclosed spaces such as closed packages or the interior of machinery during shipment. The advantage it has over a coating is reach, because the volatilised molecules get into hard to reach spaces such as a void between two metal flanges, which is exactly where a tool or an assembly rusts first.
| Measure | Why it works | Where it applies |
|---|---|---|
| Keep the packed part below the critical humidity | Corrosion needs a wet surface, and the time of wetness is the fundamental parameter. Removing the water removes the electrolyte layer | Warehouse, workshop and container |
| Vapor phase corrosion inhibitor in the package | A vapor phase inhibitor has enough vapor pressure to reach the metal through the vapor phase and forms a protective layer on the surface, so it protects without being applied directly to the metal | Closed packages and machinery interiors during shipment |
| Vapor phase inhibitor for hard to reach spaces | The volatilised molecules reach voids that a coating cannot, including the space between two metal flanges and similar enclosed gaps | Assemblies, spare parts in a crate, enclosed voids |
| Wrap and seal rather than wrap alone | A barrier only works if the pollutant cannot reach the surface, and chloride deposition is the pollutant that matters most near the coast | Sea freight and coastal storage |
| Dry the part after washing or testing | Washing supplies water and rain dilutes deposits but the surface still has to be dried, or the residue of a wash becomes the electrolyte | After a corrosion test, a dye penetrant check or a wet cleaning step |
| Remove deposits rather than loosen them | Aerosol particles absorb water and lengthen the time of wetness, so a layer of dust keeps the surface wet for longer than a clean one | Long term storage and re packed stock |
| Do not rely on passivation where chromium is not present | Passivity is a chromium effect. A carbon or low alloy tool steel has no passive region to sit in at atmospheric pH, so it has to be protected rather than made resistant | Carbon and low alloy tool steels |
Protection measures for storage and transit, ordered from the general atmosphere control to the specific use of a vapor phase inhibitor. The last row is the reason the rest of the table exists.
Two cautions belong with the inhibitor route. The source classifies inhibitors into passivating, organic and vapor phase types, and passivating inhibitors are oxidizing chemicals such as chromates, nitrites and molybdates that shift the corrosion potential several tenths of a volt in the noble direction. That is a large shift for a small addition, which is why an excess of a passivating inhibitor can push a surface into localized corrosion rather than protect it. The vapor pressure of a vapor phase inhibitor is also a working parameter, and the source treats a vapor pressure of the order of one millionth of a torr as too low to be effective. Both points argue for using a packaging product as specified rather than improvising with a general purpose chemical.
How a delivery condition is checked
Where a purchase order needs a number rather than an assurance, the route is a coupon exposure and a mass loss, and the standards for it are old and detailed enough to remove most of the argument. ASTM G 50 covers conducting atmospheric corrosion tests on metals, ASTM G 4 covers corrosion coupon tests in field applications, and ASTM G 1 covers preparing, cleaning and evaluating the specimens afterwards, which is the step where a result is most easily spoiled. ASTM G 92 covers how an atmospheric test site is characterized, and ASTM G 84 covers measuring the time of wetness directly on the surface. On the pollution side, ASTM G 91 covers monitoring sulfur dioxide with a sulfation plate and ASTM G 140 covers determining the atmospheric chloride deposition rate by the wet candle method.
| Standard | Subject |
|---|---|
| ISO 8565 | Metals and alloys, atmospheric corrosion testing, general requirements for field tests |
| ISO 9223 | Corrosion of metals and alloys, corrosivity of atmospheres, classification |
| ISO 9225 | Corrosion of metals and alloys, corrosivity of atmospheres, measurement of pollution |
| ISO 9226 | Corrosion of metals and alloys, corrosivity of atmospheres, method of determination of corrosion rate of standard specimens for the evaluation of corrosivity |
| ISO 8407 | Corrosion of metals and alloys, removal of corrosion products from corrosion test specimens |
| ISO 11463 | Corrosion of metals and alloys, evaluation of pitting corrosion |
| ISO 9227 | Corrosion tests in artificial atmospheres, salt spray tests |
| ASTM G 50 | Standard practice for conducting atmospheric corrosion tests on metals |
| ASTM G 4 | Standard guide for conducting corrosion coupon tests in field applications |
| ASTM G 1 | Standard practice for preparing, cleaning and evaluating corrosion test specimens |
| ASTM G 92 | Standard practice for characterization of atmospheric test sites |
| ASTM G 84 | Standard practice for measurement of time of wetness on surfaces exposed to wetting conditions |
| ASTM G 91 | Standard practice for monitoring atmospheric sulfur dioxide using the sulfation plate technique |
| ASTM G 140 | Standard test method for determining atmospheric chloride deposition rate by the wet candle method |
| ASTM G 101 | Standard guide for estimating the atmospheric corrosion resistance of low alloy steels |
| ASTM G 46 | Standard guide for examination and evaluation of pitting corrosion |
| ASTM G 60 | Standard test method for conducting cyclic humidity tests |
The standards the source lists for characterizing an atmosphere and for running atmospheric corrosion tests, given here as a working list for a supplier who has to state what a delivery condition was tested against. Condensed from Table 2 of the article.
Read that list as the specification behind a shipping claim. If a supplier states that a delivery was packed for a corrosivity category, the two numbers in the category have to come from somewhere, and those standards are where. Removal of the corrosion products before weighing the coupon is covered separately because it is the operation that decides whether a light rust film is counted or dissolved away, and it is the reason a mass loss figure quoted without the corrosion product removal method is not comparable with another one. The specification side of the same question is on tool steel bar delivery condition and tool steel supply, the surface state a customer receives is described on bar macroetch and carbide segregation, and the tests that are run on the steel itself are on how steel chemistry is tested and steel sample taking for a lab test.
What this means on a tool steel order
Three things follow. Ask where the material will be stored and shipped, because the two deposition rates and the time of wetness decide the packaging requirement more than the grade does. Treat a lightly condensed surface as the dangerous case rather than the visible wet one, since the thin film is saturated in oxygen and corrodes faster than a heavier layer. And separate the two product groups in the packing standard, because a stainless grade can be selected for the environment while a carbon or low alloy grade can only be protected from it. Where the environment is coastal the pitting mechanism behind chloride attack is covered on pitting corrosion in tool and mold steel, the same mechanism inside a closed gap is on crevice corrosion in mold steel, and the grades that earn their corrosion resistance from chromium are described on 440C stainless steel and 420 stainless steel.
Before you act on a rust complaint
A reference page, it is not an Aobo Steel specification, and the thresholds, deposition rates and standards above are the values published for the environments the source describes. Whether a particular store or route rusts a package depends on the actual temperature and humidity record, the chloride and sulfur deposition at that site, the packing method and the time in transit, none of which can be read off a single photograph of the surface. Treat the classifications as a framework for choosing a packing standard, and confirm the surface condition on arrival against an agreed method rather than against a description.
Source: ASM Handbook, Volume 13A, Corrosion: Fundamentals, Testing, and Protection, ASM International, 2003.