THE SALT FOG CORROSION TEST
WHY DO A SALT FOG CORROSION TEST?
The Salt Fog is a standardized laboratory test aimed at verifying the corrosion protection of a metallic material, its organic / inorganic coating or surface treatment. It is nothing more than an accelerated corrosion test and as such it provides for a more aggressive environment than that foreseen in operation to accelerate the corrosive processes and reduce the duration of the tests compared to those carried out in natural conditions. Corrosion tests simulate environments where there is a salt concentration, such as seashores, cliffs and even boats.
The duration of a salt spray test can vary from a few hours to many hundreds (even beyond 1000 hours). The specifications or customer specifications indicate a certain number of hours of exposure for each type of material/coating, below which there must be no signs of corrosion or in any case within a certain limit. There is not always a direct correlation between salt spray resistance and behavior in other environments. Above all because many factors that influence the progress of corrosion (for example the formation of protective layers) vary according to the environmental conditions and the stresses encountered in practice.
Corrosion tests are widely used in the industrial field. Corrosion, in addition to causing aesthetic damage with, for example, the appearance of the well-known “rust” (iron oxidation phenomenon) or “white corrosion” (corrosion of the zinc plating), can also lead to structural or mechanical damage, thus causing problems level of safety and functionality/resistance of components.
TECHNICAL ASPECTS OF SALT FOG TEST
The sample is placed in the corrosion chamber where the saline solution is sprayed through a nozzle and the “FOG” is created. The sample is not directly hit by the nebulised jet and must be inclined with respect to the vertical of about 15 – 30° (variable angle according to the standard/method). When these tests are required on flat plates, it is easy to impose their inclination, but in most cases we are dealing with three-dimensional samples such as car rims, shock absorbers, small parts such as screws and bolts, etc.… With three-dimensional samples it is not it can certainly confirm the correct inclination in all points of the exposed surface. In these cases, the positioning of the sample inside the chamber is agreed with the customer, trying to respect the indicated inclination as much as possible.
The various salt fog tests (Neutral, Salina Acetica or Cupro Salina Acetica) differ substantially in the solution and in the temperature inside the chamber.
Below, taking the ISO 9227 standard as an example, a summary table and some details about the solution will help you understand the differences:
|
PARAMETER |
NEUTRAL SALT FOG (NSS) | ACETIC SALT FOG
(AASS) |
CUPRO ACETIC SALT FOG (CASS) |
| CHAMBER TEMPERATURE | 35± 2 °C | 35± 2 °C | 50± 2 °C |
| SODIUM CHLORIDE CONCENTRATION
(NaCl) |
50 g/l ± 5 g/l | 50 g/l ± 5 g/l | 50 g/l ± 5 g/l |
| SOLUTION COLLECTION SPEED |
1,5 ml/h ± 0,5 ml/h |
||
| pH (collection solution)
at 25± 2 °C |
6.5 – 7.2 | 3.1 – 3.3 | 3.1 – 3.3 |
SALT SPRAY REGULATIONS
The salt fog test was born in 1914 in the United States at the National Bureau of Standards (NBS) now known as NIST. The procedure was formalized by ASTM International, an organization that develops and publishes standards, in 1939 with the code ASTM B117. The salt fog test continues to be included in the test specifications of many industries. The method was subsequently taken up by General Motors which lowered the concentration from 200 to 50 g/l NaCl (1954), significantly improving its management and reproducibility. Numerous specifications have since described this test.
Below we indicate today the various salt spray tests with some international specifications and fields of application:
- Neutral Salt Fog (NSS), whose reference standards are for example ASTM B117 and ISO 9227. Fields of application:
– metals and related alloys;
– metallic coatings (anodic and cathodic);
– conversion coatings;
– anodic oxidation coatings;
– organic coatings on metallic materials.
- Acetic Salt Fog (AASS) regulated for example by ASTM G85-A1 and ISO 9227. Fields of application:
– decorative coatings of copper + nickel + chromium;
– decorative coatings of nickel + chromium;
– anodic coatings on aluminium.
- Cupro Saline Acetic Fog (CASS) whose test methods are reported for example in ASTM B368 and ISO 9227. Fields of application:
– decorative coatings of copper + nickel + chromium;
– decorative coatings of nickel + chromium;
– anodic coatings on aluminium.
THE SOLUTIONS
Neutral Salt Mist 5% NaCl Solution is then used.
Acetic Salt Fog (AASS) to the salt solution (NaCl 5%) sufficient glacial acetic acid must be added until a pH between 3.0 and 3.1 is reached. The collection solution should have a PH between 3.1 and 3.3 at 25°C (see table).
Cupro Saline Acetic Acid Mist (CASS) a sufficient amount of copper chloride should be added until a concentration of 0.26 g/l ± 0.02 g/l is reached (pH as per AASS).
EVALUATIONS AFTER THE SALT FOG TEST
The criteria applied for the evaluation of the results are many, the most common is the evaluation of the appearance of the sample at the end of the test.
Some evaluation criteria are:
- Number and distribution of defects caused by corrosion;
- Appearance after removal of surface corrosion products;
- mass loss;
- Micrographic examination;
- Variation of mechanical characteristics.
There are specific methods to do this, for example:
-ISO 8993 assessment of pitting in anodized aluminium;
-ISO 10289 evaluation of metallic and inorganic decorative protections.
On organic coatings (paints) engraving with a St Andrew’s cross (“X”) or a single engraving is often required before exposure to the Salt Mist. At the end of the test, the evaluation of the corrosive propagation from the engraving becomes the parameter to be evaluated for the conformity or otherwise of the product. Often the method for verifying the propagation is by tearing with adhesive tape (e.g. TESA 4657)
Technical evaluations on coatings often refer to other international standards such as, for example, ISO 4628 (evaluation of the protection of organic coatings.) Here are some of them:
- BLISTERING in reference ISO 4628-2
- RUSTING in reference ISO 4628-3
- CRACKING in reference ISO 4628-4
- FLAKING in reference ISO 4628-5
- DELAMINATION AND CORROSION AROUND A SCRIBE in reference ISO 4628-8
- FILIFORM CORROSION in reference ISO 4628-10
THE IMPORTANCE OF QUALITY IN CORROSION TESTING
The quality and correctness of the result is the result of a constant work of checking all the chamber parameters, calibrations and intermediate checks.
To verify the repeatability of the results of a salt spray chamber and the reproducibility of the results of salt spray chambers in different laboratories it is necessary to check the corrosivity through standardized reference specimens.
In our laboratories this is a procedure that we carry out on a regular basis in order to always operate in the correct way.
In short, as Quality Manager of your company that deals with surface coatings, the Salt Fog test is the only way you have to declare the quality of your products to your customers.
And what better way to carry out this test than to contact an ACCREDIA Accredited Laboratory, according to ISO/IEC 17025, precisely for the salt spray test according to ISO 9227 and ASTM B117-19?
The accelerated aging tests are constantly monitored by Expert Operators, so as to be able to warn you immediately in the event of non-compliance and decide together how to proceed.
In the event of an anomalous outcome of the Salt Fog test, however, our Personal Technician can advise you on additional tests to investigate the anomalous appearance of white or red corrosion. The only person able to satisfy your every need, following your requests step-by-step: from the request for an offer, up to the issue of the report and, if necessary, even after.
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