Corrosion Testing Methods for Steel

What an accelerated test can and cannot do

The source is direct about this, and the warning is the most useful sentence in the whole subject. Salt spray tests have been used for over a hundred years as accelerated tests for the corrodibility of ferrous and nonferrous metals and for the degree of protection that inorganic and organic coatings give. They are used most often to compare the relative performance of metals and coatings against a known and retained standard, for quality control. The reproducibility variances have been discussed at length, and so has the questionable correlation of the results with actual service performance, and the source names the cause. The test was never intended to simulate a particular environment or service condition, except in the rare cases where corroborating data exist to support that use. What it actually applies is a standardised amount of corrosive activity so that materials and coatings can be compared against each other.

That framing decides how a corrosion result should be read on a steel order. A salt spray figure supports a statement about consistency and relative position. It does not support a statement about how long a mold will last, and the difference matters most in the cases where the customer is asking the second question and the supplier is answering with the first.

Cabinet tests and the conditions they run at

The most commonly used salt spray methods in the United States and much of the rest of the world are the ones in ASTM B 117, ASTM B 368 and ASTM G 85, and many national and company standards largely conform to those details. The same apparatus, properly equipped, also runs the humidity tests, which use deionized water as the only corrosive agent and carry no salt at all. The table below collects the methods with their electrolyte, pH, temperature and step structure, including the cyclic tests that were developed to bring the exposure closer to an outdoor cycle by alternating wet, dry and salt stages.

Commonly used cabinet corrosion test methods as the source tabulates them, grouped into the static tests and the cyclic tests. The humidity tests use deionized water as the only corrosive agent and carry no salt. Condensed from Table 1 of the article.

Two readings of that table are worth keeping. The first is that the static neutral fog methods all sit at the same place, 5% sodium chloride at a pH between 6.5 and 7.2 and a temperature of 35 C, and the differences between the national standards are mostly in minor tolerances. The acidified variants move the pH to between 2.8 and 3.3, and the copper accelerated variant adds 0.25 grams per litre of copper chloride and raises the temperature to 49 C, which is how the test is made faster without changing the apparatus. The second reading is that a cyclic test is not a harsher version of a static one. It is a different experiment, and a ranking from one of the cyclic methods will not necessarily match a ranking from the static fog even though both are called salt spray.

Electrochemical methods and what they add

Electrochemical methods are the route to a number rather than a ranking. The basis is that the polarisation resistance, defined as the slope of the polarisation curve at the corrosion potential where the current is zero, is inversely related to the corrosion current density, and the relation is the Stern and Geary equation. The corrosion current density equals a constant B divided by the polarisation resistance, and B is calculated from the anodic and cathodic Tafel slopes. Where the slopes are not known, the source notes that a constant B between 13 and 26 millivolts is often used for qualitative estimates, and that is the source of most of the disagreement between two laboratories reporting different rates for the same steel.

Electrochemical methods for corrosion testing as the source presents them. The small signal methods are described as nondestructive, which is what allows them to be repeated on the same specimen.

Two experimental effects decide whether a resistance measurement is worth anything. The first is scan rate. If the potential is swept too fast, the measured polarisation resistance comes out too low and the corrosion rate is overestimated. The second is the uncompensated resistance between the reference electrode and the surface, which adds itself to the measured value because the experimental resistance contains both terms. That error can be significant in a system with a high corrosion rate in a poorly conducting solution, and it can be removed during the measurement by positive feedback or current interruption, or subtracted afterwards. Both effects point the same way in practice. A rate quoted from a polarisation resistance measurement without the method of potential control and the treatment of ohmic drop is a figure that cannot be compared with another one, even on the same steel.

The small signal methods have one property that matters for a supplier. Because only a small signal is applied so that the system stays linear, the measurement is nondestructive and can be repeated many times on the same specimen. That is what makes continuous monitoring possible, and the source gives the main advantage of the polarisation resistance route over periodic weight loss as the ability to follow an instantaneous rate continuously rather than to sample it. Impedance spectroscopy goes further and describes the system at a fixed working point across a very wide frequency range, typically from 100 kHz down to 1 mHz, which is why it is presented as a Bode plot rather than as a single resistance and why an equivalent circuit model is needed to turn a spectrum into a rate.

Putting two rates side by side

Corrosion rates are reported in units that cannot be compared directly, because some are mass based and some are penetration based. The conversion between them needs one extra quantity, the density of the metal, since a mass loss per unit area only becomes a depth of metal lost when the density is known. The table below gives the full set of relationships for the units in common use, and the density term appears as the symbol d in the factors that need it.

The conversion factors between the units commonly used for corrosion rates, read across from the unit in the left column. The symbol d stands for the density of the metal in grams per cubic centimetre, which is why the mass based units cannot be converted to a penetration rate without it. Source, Table 1 of the corrosion rate conversion section.

For tool steel work the practical consequence is simple. A rate quoted in mils per year from an American source and a rate quoted in millimetres per year from a European one have to be brought to the same unit before any comparison, and a rate quoted in milligrams per square decimetre per day has to be divided by the density of the steel as well. The conversion itself is not the hard part. Deciding which rate belongs to the surface condition in front of you is, because a mill finish and a ground finish of the same grade do not test the same.

Which test answers which question

It is worth separating the question types, because the wrong test is usually chosen because the right question was never written down. A supplier asking whether a heat is consistent with the last one needs a ranking test against a retained standard, which is what a salt spray cabinet or a ferric chloride exposure gives. An engineer asking how long a component will last needs either field data from the same environment or a repassivation potential used as an operating limit, and neither of those is a cabinet test. A process engineer asking whether a cooling circuit is currently corroding needs a continuous method rather than a coupon, which is the polarisation resistance or resistance monitoring route.

What each class of corrosion test can and cannot support, taken from the way the source states the purpose of each one. The governing idea is that an accelerated test ranks materials and confirms a condition, and does not predict time to failure.

One limit runs across the whole table and is worth stating on its own. Averaging a uniform corrosion rate is exactly the operation that hides localized attack, because a pit that perforates a wall can consume a negligible mass of steel. That is why the source describes the on line electrical and electrochemical methods as more suited to monitoring uniform corrosion than localized corrosion, and why a pitting assessment has to be a separate examination. The pitting side of that is covered on pitting corrosion in tool and mold steel, and the crevice version of the same examination on crevice corrosion in mold steel.

What this means on a tool steel order

Three things follow. Name the test as well as the requirement, because salt spray, ferric chloride and a polarisation scan will rank the same three grades in different orders. State the density alongside any mass based rate, or state the rate as a penetration depth and avoid the conversion entirely. And keep the localized corrosion check separate from the uniform rate measurement, because the two are answering different questions and the second does not detect the first. The general inspection and verification route that this sits inside is on tool steel quality verification, how steel chemistry is tested and steel sample taking for a lab test, the specimen preparation side that decides most of these results is on hardness testing methods and inclusion and grain size measurement, the grades and their corrosion behaviour are charted on the stainless steel corrosion resistance chart, and the packing standard that a corrosivity classification feeds into is described on rust prevention in storage and transit.

Source: ASM Handbook, Volume 13A, Corrosion: Fundamentals, Testing, and Protection, ASM International, 2003.