Introduction
Steel tubes used in service environments could face exposure to moisture, salts, and other corrosive influences. Surface treatments and coatings on steel may provide protection to the underlying material, however their performance must be evaluated against the relevant requirements.
Salt spray test of steel tubes is a laboratory practice that exposes a specimen to a controlled salt-fog environment for a defined period, observing how it responds. Comparing surface finishes and identifying coating inadequacies under the test conditions can be informative, however the outcome must not be extrapolated to all environments of use.
Salt spray testing of steel tubes provides a controlled method for evaluating surface protection under specified test conditions.
What Is Salt Spray Testing?
Salt spray testing or the salt fog test uses a prepared specimen to expose it in a test chamber to a defined atmosphere with a salt solution.
The conditions of the exposure, preparation of the specimen, duration of the test, inspection methods and requirements are determined by the applicable test standard used or a product specification. Following exposure the specimen is examined for defined corrosion or coating degradation manifestations according to the procedures of the chosen test method.
The practice is applied in assessments of coated or treated metal surfaces, and a suitable test method needs to be selected on the surface treatments under examination.
How Does Salt Spray Corrosion Test Work?
A salt spray corrosion test typically works by preparing the specimen, setting up a salt spray chamber and exposing it for the defined duration of the test. Following exposure it is visually inspected according to criteria defined by the method or specification with observations noted and reported in accordance with the acceptance requirements.
The preparation for the test and evaluation criteria will be key to its outcome, and the results of different procedures should not be compared if using different test methods.
What Can Salt Spray Testing Reveal About a Specimen?
A salt spray test will indicate if corrosion manifestation or protective treatment degradation occurs under the test conditions. The observations for passivation or coating treatments will depend on the coating type and the specification's acceptance requirements. Possible results may include:
- Observation of base metal corrosion.
- Appearance of corrosion products on the treated surface.
- Signs of localised treatment deficiencies or
- Blistering if relevant for the coating type.
- Deterioration of the coating or appearance of defects at relevant edges, openings or coating discontinuities.
Application of a test method will indicate whether a result is conforming or otherwise, and should not be considered for a conclusion other than what the requirements state.
How Is Corrosion Testing of Steel Tubes Used?
Corrosion testing of steel tubes, such as a salt spray test, helps in assessing the performance of a material or surface treatment in a prescribed manner. A salt spray test is one option for testing, however there are other test methods available that replicate other environments of use such as the presence of specific chemicals, high temperature, immersion and cyclic exposure with wetting and drying.
The test should represent the requirement as much as possible within parameters defined by the applicable test method.
Results obtained by exposure to salt spray assist with comparisons of a similar test procedure against a stated specification, however each test is to be assessed according to the conditions of the procedures and the acceptance criteria.
What Is Passivation Testing?
Passivation test procedures assess the suitability of a passivated surface for its intended purpose. The method of examination depends on the surface condition, the treatment and the objective of the assessment.
For relevant grades of stainless steel, appropriate checks can assess contamination of the surface or condition of the passive layer. Where a salt spray test is specified it should still be evaluated as to whether it applies to the material, surface condition and product specification.
The acceptance requirements should be defined prior to conducting a passivation test. A method that applies for one alloy or treatment may not be suitable for another.
How Does Salt Spray Testing Fit into Protective Surface Treatment Testing?
Protective surface treatment testing comprises more than simply a salt spray test. For certain types of coatings, a suitable quality plan for the protective treatment may require a visual examination of the treatment or coating, examination of coating thickness (where applicable), adhesion testing (where applicable) verification of treatment or coating records, salt spray exposure and assessment against defined criteria as required by a specification.
The suitable combination will depend on the treatment, material, application and purchase specification for the protective surface treatment.
What Are the Limitations of Salt Spray Testing?
A salt spray test represents controlled comparative testing for determining coating performance, or for demonstrating a conformity to a specified standard. The performance of a coating system when being used or exposed to other environments will also depend on the influences of temperature, mechanical stresses, chemical exposure, maintenance, and frequency of wetting and drying.
Duration of a salt spray test therefore should not be confused with duration of service life. The outcome of a salt spray test can only be compared with other salt spray tests according to the methods used, and within a test method's scope of service life.
How Does Sainest Tubes Address Inspection and Surface Protection?
Sainest Tubes Pvt. Ltd. makes information available on tube inspection, testing and protective finish procedures. Suitable protective treatment verification methods can be determined by considering the material, type of treatment, purchase specification and customer requirements.
Conclusion
Salt spray exposure is an assessment for observing the effects on material or coating condition. Its application is best applied when suitable specimen preparation, test procedure and acceptance criteria are defined in accordance with a suitable specification and test method. In conjunction with related protective treatment assessment procedures, it can assist in ensuring steel tubes' quality without over-reliance on a single method for the suitability for all applications.