Salt Spray and Corrosion Tests

If the salt spray report for a batch of chassis says 500 hours, you cannot derive from that figure exactly how long the part will last in the field. The test was not designed for that. Salt spray exists to put two different pretreatment recipes or two coating systems side by side under the same conditions and see which one fails later. Many read the number like a guarantee; few know what it measures.

Why Test

Outdoors, corrosion takes months, often years. You cannot wait three years to see the result of a decision you made on a coating line. Salt spray compresses that time into a cabinet: it accelerates corrosion with a continuous salt fog, constant temperature and humidity. So you get a result within weeks.

The most solid use of the test is comparison. For example, you can see in the same cabinet over the same period which pretreatment — iron phosphate or nano-ceramic — gives better adhesion and corrosion protection. Or you can measure how much difference a duplex system applied over galvanizing makes compared with powder coating alone. What is valuable is the relative difference, not an absolute “service life.”

The second use is process control. The pretreatment bath becomes exhausted over time, rinse conductivity drifts and conversion coating weight drops. Placing samples and testing them at set intervals lets you catch the drift before the line starts producing bad parts. In practice, most plants run a standard test panel (e.g. 100 x 150 mm cold-rolled steel sheet) through the line at every color change or weekly and place it in the cabinet. Because these panels see the same pretreatment and the same coating, any deviation in the result comes from the process, not the part.

A third use is the approval test performed before introducing a new resin or a new pretreatment chemical. Decisions such as changing suppliers, switching to a new conversion coating or moving to low-temperature curing are verified with salt spray before entering series production.

How the Test Is Done

The most common standard is ASTM B117 (neutral salt spray, NSS). The cabinet keeps the following conditions constant:

Sample preparation directly affects the result. A controlled scratch (scribe) reaching down to the metal is made in the coating surface. The real purpose of the test is to see what happens around this scribe; an intact surface fails late anyway, and the critical area is the point of damage. Edges and hanger contact marks are usually masked, because these are artificial weak points.

ASTM B117 is a neutral fog test. Its derivatives are also used for more aggressive evaluations: AASS, in which pH is lowered to ~3.2 with acetic acid, and CASS, with added copper chloride, create harsher conditions, especially for decorative and exterior coatings. In the automotive industry, cyclic tests (with wet/dry cycles, CCT; e.g. ISO 11997 or OEM standards) are preferred over constant fog. Because cyclic tests include wet-dry transitions, they behave somewhat closer to the field than B117, but they still do not give a one-to-one service life.

In choosing the duration, the environment the part will be exposed to is decisive. While 240 hours may be considered sufficient for an indoor cabinet body, 1000 hours or more may be required for a galvanizing-powder coating duplex part exposed to a coastal environment or road salt. Rather than choosing the duration arbitrarily, it is more correct to base it on the target corrosivity category (e.g. ISO 12944 C3–C5).

Interpreting the Results

At set hours, the parts are removed, rinsed and two things are evaluated:

• Creep from the scribe: how far rust has advanced on both sides of the scribe line is measured in millimeters. The coating separating from the metal with corrosion advancing underneath, i.e. delamination, is read here.
• Surface deterioration: blistering, rust spotting, pinholes and general corrosion in the area outside the scribe are rated. Visual scales such as ASTM D714 for blistering and ASTM D1654 for degree of corrosion are used.

Early blistering on an intact surface usually points to an adhesion problem before corrosion; the root cause is most often in pretreatment. If degreasing is inadequate, rinsing is poor or the conversion coating is too thin, the film adheres not to the metal but to a contaminated interlayer and lifts quickly under moisture. If there is only limited creep at the scribe and the surface is clean, the coating system is doing its job.

That is why the salt spray report must be read not on its own but together with adhesion tests such as cross-cut (ISO 2409). The two tests answer different questions: cross-cut says “how well does the film adhere to the metal,” salt spray says “how long does this adhesion last under moisture and salt.” When the two conflict, for example good adhesion but low corrosion resistance, the problem usually lies in the corrosion performance of the conversion coating, not in mechanical adhesion.

Who performs the evaluation, and with which scale, also affects the result. Blister density and size are classified with visual comparison scales and carry a degree of subjectivity depending on the observer. That is why using the same scale, the same lighting and, if possible, the same evaluator in comparisons between batches preserves the meaning of the number.

To give a threshold: for architectural aluminum, many specifications require 1000 hours NSS, and for general exterior steel, 500 hours is a frequently requested target. However, the acceptance criterion depends on where the part will be used and on the customer’s specification; the exact value is defined in the project specification.

Test vs Real-World Field

The biggest misconception about salt spray is conversions like “500 hours = 5 years.” No such conversion factor exists. The test creates a continuous, uniform environment: always wet, always salty, always 35 °C. In the real atmosphere, wet-dry cycles, UV, temperature fluctuations and pollution act together. Wet-dry transitions in particular trigger corrosion in a way B117 cannot imitate at all.

It pays to be cautious with a supplier who presents a high salt spray figure on its own as a marketing argument. The test is strong for comparing two systems; it is weak for promising the field life of a single system. The question “which standard, which sample preparation, which rating scale was used” matters as much as “how many hours did it last.” The same part can produce a better result on paper if the scribe is cut more shallowly or the edges are masked more generously.

These limits do not make the test useless. Because salt spray holds most variables constant, it is valuable precisely as a tool for controlled comparison and process monitoring. Just read the result for what it is: an indicator of relative corrosion resistance, not a certificate of warranty. Real field life becomes clear only through observation under real conditions, correct surface preparation and the choice of a suitable coating system.