For over a century, humankind has waged a relentless war against the stealthy menace of corrosion. This silent adversary eats away at structures and machines, weakening them.
To combat this threat, scientists have devised ingenious methods to test materials’ resilience, with one pivotal technique leading the charge: salt spray corrosion testing.
Through a storied evolution, this testing has become a vital weapon in our arsenal, helping guarantee products that withstand the tests of time.
Series: Salt Spray Testing
This is the second article in a series of three that explores aspects of salt spray testing:
- The Silent Adversary: The never-ending quest to improve salt spray corrosion testing reflects our determined spirit of triumph over the cunning foe, corrosion.
- Unveiling the Secrets of Salt Spray Testing: A Comprehensive Guide: Learn all about salt spray testing—how it’s done, its pros and cons, and a glimpse into the world of corrosion protection.
- Deja Vu All Over Again: Cyclic Corrosion Testing: Cyclic Corrosion Testing enables proactive protection against corrosion’s stealthy strikes by revealing risks when mitigation is most effective.
Early Beginnings of Salt Spray Testing
In the early 20th century, corrosion testing of materials and coatings became an important need across industries. This testing helped evaluate how well products would withstand harsh environments.
In 1914, a key development in corrosion testing occurred when J.A. Capp proposed using a neutral salt spray method. His proposal marked the genesis of what we now call Salt Spray Testing. This technique has been widely used for over a century to assess corrosion resistance.
Capp’s recommendation to use a neutral salt spray laid the groundwork for a vital evaluation tool still used today. His contribution enabled the standardized testing of products against corrosion.
While simple, Capp’s salt spray proposal shaped a universal practice in durability testing.
Meet ASTM B117
In 1939, a significant milestone in salt spray corrosion testing was reached with the introduction of the Neutral Salt Spray Test, also known as ASTM B117. This standardized test method revolutionized the field by providing a consistent and reproducible way to evaluate the corrosion resistance of various materials and coatings.
The development of ASTM B117 was driven by the need for a controlled environment to simulate real-world corrosive conditions.
The Challenge Before ASTM B117
Before this test, there were no uniform criteria or procedures for evaluating corrosion resistance, leading to inconsistent results and making it difficult to compare different materials or coatings. The Neutral Salt Spray Test involved subjecting specimens to a continuous spray of 5% sodium chloride (NaCl) solution at 95 ºF (35 °C).
Test Parameters and Conditions
The solution’s pH level was set between 6.5 and 7.2, ensuring it neither promoted nor inhibited corrosion. This neutral pH range allowed for an accurate assessment of material performance with no bias towards acidic or alkaline environments.
Strict Guidelines for Testing
ASTM B117 established strict guidelines for test chamber design, specimen preparation, exposure duration, and evaluation criteria to ensure reproducibility and comparability across different laboratories. These guidelines aimed to minimize variations in testing conditions and procedures that could influence test results.
Evaluation and Monitoring Methods
Upon exposure to the neutral salt spray environment, materials were carefully monitored over predetermined intervals to assess their resistance to corrosion. The evaluation typically involved:
- Visual inspection
- Measuring of corrosion products formed on specimens’ surfaces
- Determining any changes in appearance or integrity
Impact on Material and Coating Technologies
ASTM B117 quickly became one of the most reliable methods for evaluating corrosion resistance. Its adoption facilitated better material selection and fueled advancements in coating technologies as manufacturers sought to enhance their products’ performance under corrosive conditions.
Updates and Refinements
Over time, ASTM B117 has undergone several updates and refinements based on ongoing research findings and industry feedback. These revisions have addressed concerns such as improved control over operating parameters, enhanced test specimen preparation techniques, and more accurate evaluation methods.
Current Significance of the Test
The Neutral Salt Spray Test remains widely used today, serving as a fundamental tool in corrosion testing across various industries, such as automotive, aerospace, marine, and construction. Its importance cannot be overstated as it contributes significantly to developing durable and corrosion-resistant materials that withstand harsh environmental conditions.
Modifications and Refinements to B117 since 1939
Over the years, the Neutral Salt Spray Test (ASTM B117) has undergone several modifications and refinements to enhance its reliability and accuracy in assessing corrosion resistance. One notable modification is the introduction of a temperature control feature.
Introduction to Temperature Control
Initially, the test was conducted at an ambient temperature, which posed limitations as environmental conditions varied across different testing facilities. A controlled temperature chamber was developed, allowing for consistent testing conditions and reducing the impact of temperature fluctuations on results, which addressed this issue.
Incorporating Cyclic Exposure
In response to concerns about the test’s ability to accurately simulate real-world corrosion conditions, additional modifications were made to ASTM B117—one significant refinement involved incorporating cyclic exposure into the testing procedure.
This “Cyclic Prohesion Test” aimed to replicate realistic environmental cycles by subjecting specimens to alternating salt spray exposure and drying periods. By introducing these cycles, researchers sought to mimic the natural occurrence of moisture condensation followed by evaporation—a phenomenon often observed in real-world corrosive environments.
Focus on Specimen Preparation
Another crucial refinement introduced in recent years is related to specimen preparation techniques. It was recognized that improper surface preparation could significantly impact test results by affecting adhesion between coatings and substrates.
ASTM International revised its standards regarding specimen cleaning methods and established guidelines for proper surface preparation techniques before conducting salt spray tests.
Standardizing Testing Equipment
Efforts have been made to standardize salt spray corrosion testing equipment to ensure consistency among different testing laboratories worldwide.
Refining Apparatus Specifications
Specifications such as nozzle designs and airflow rates have been refined over time to minimize variations between different apparatuses employed by various laboratories. These modifications and refinements illustrate the continuous efforts dedicated to enhancing the reliability and accuracy of salt spray corrosion testing since its inception in 1939.
By improving temperature control mechanisms, incorporating cyclic exposure patterns, refining specimen preparation techniques, and standardizing equipment specifications, researchers have strived to create a more accurate simulation environment for evaluating the corrosion resistance of materials.
These advancements have undoubtedly contributed to the growth of understanding and knowledge in the field, enabling industries to develop more effective and durable corrosion-resistant coatings and materials.
The Cyclic Prohesion Test
The Cyclic Prohesion Test is a specialized salt spray test developed to simulate the cyclic nature of corrosion in real-world environments.
It was introduced as an enhancement to the traditional salt spray testing method, which only exposed samples to a continuous salt mist. Unlike the steady-state conditions of the salt spray test, the Cyclic Prohesion Test subjects specimens to alternating wet and dry cycles, more closely replicating the dynamic conditions experienced by materials in nature.
How the Test Works
This test exposes samples to a saline solution for specific durations, followed by drying periods under controlled conditions. This wet-dry cycling is achieved by intermittently spraying a salt solution onto the specimens and then allowing them to air dry completely before repeating the process.
The frequency and duration of wet and dry cycles are adjusted based on the intended application or desired level of corrosion resistance being evaluated.
Importance of Cyclic Exposure
The significance of incorporating cyclic exposure lies in accelerating corrosion mechanisms, such as galvanic corrosion, crevice corrosion, and stress corrosion cracking.
These degradation processes commonly occur because of variations in moisture content on material surfaces. By subjecting samples to cyclic wetting and drying conditions, researchers better analyze how different materials respond when exposed to fluctuating moisture levels and corrosive agents.
Real-World Simulation Advantages
One notable advantage of using the Cyclic Prohesion Test is its ability to provide more reliable data on material performance than traditional static tests. The real-world conditions simulated during this testing method enable engineers and scientists to accurately assess how different coatings or treatments will withstand cyclical environmental changes over extended periods.
Because of its effectiveness in evaluating long-term durability against cyclic corrosion phenomena, many industries rely on the Cyclic Prohesion Test when developing protective coatings for various applications such as automotive parts, marine equipment, infrastructure components like bridges or pipelines, and even electronic devices that need protection from harsh environments.
Comparison with Traditional Testing
While conventional salt spray testing provides valuable insights into the initial corrosion resistance of materials, the Cyclic Prohesion Test is a significant advancement that better mimics real-world conditions.
By subjecting specimens to cyclic wet and dry cycles, this testing method offers a more comprehensive understanding of how materials perform in dynamic environments, allowing for more accurate predictions of long-term corrosion behavior. This enhanced evaluation has proven essential for industries seeking durable and reliable materials that withstand the challenges posed by cyclic corrosion mechanisms.
Studies Improving Salt Spray Testing
The Society of Automotive Engineers (SAE): The Society of Automotive Engineers (SAE) has significantly improved salt spray corrosion testing.
SAE’s Impact on Salt Spray Testing
In the 1950s, SAE established a corrosion test committee to address the growing concerns in the automotive industry regarding the durability and lifespan of various components exposed to harsh environments. Through extensive research and collaboration with industry experts, SAE developed comprehensive standards and test methods for evaluating corrosion resistance.
Performance-Based Tests by SAE
One notable contribution by SAE is the development of performance-based tests that assess not only the ability of materials to resist corrosion but also their ability to maintain functionality under corrosive conditions. These tests go beyond simple visual inspections or weight loss measurements by evaluating electrical conductivity, mechanical strength, or surface contact resistance.
The American Iron and Steel Institute (AISI): The American Iron and Steel Institute (AISI) has also played a crucial role in advancing salt spray corrosion testing, especially concerning steel products.
AISI’s Role in Corrosion Testing
Recognizing that steel is extensively used in various industries and applications, AISI conducted many studies to determine effective measures for enhancing corrosion resistance. Through rigorous experimentation and analysis, AISI developed guidelines for specific types of steels used in different environments or applications prone to aggressive atmospheres or corrosive agents.
AISI’s Research Focus
This comprehensive approach enabled manufacturers to select appropriate steel grades based on their intended usage scenarios, ensuring prolonged durability and safety. AISI’s research focused on understanding the underlying mechanisms behind steel deterioration when subjected to salt spray environments.
Key Factors Identified by AISI
By identifying key factors influencing corrosion rates—such as alloy composition, surface treatment methods, and coating thicknesses—AISI’s findings became invaluable for designing efficient anti-corrosion strategies.
Contributions of Both Organizations
Both SAE and AISI have made remarkable contributions in advancing salt spray corrosion testing methodologies through their extensive studies.
Their efforts have led to standardized evaluation criteria and provided valuable insights into material behavior under corrosive conditions. These studies have significantly benefitted various industries, particularly automotive and steel, by enhancing product durability and reducing maintenance costs.
Japan’s Contribution to CCT Methods
The Japanese have significantly contributed to developing different Corrosion Cyclic Test (CCT) (pdf) methods in salt spray corrosion testing.
Mixed Flowing Gas Test
One prominent method developed in Japan is the Mixed Flowing Gas (MFG) test. This method involves subjecting samples to salt spray exposure and gaseous corrosive environments.
The MFG test simulates real-world conditions more accurately by introducing various gases, such as sulfur dioxide, nitrogen dioxide, hydrogen sulfide, and salt spray. These gases are prevalent in industrial atmospheres where corrosion is a concern.
Benefits of the MFG Test
The MFG test provides researchers valuable insights into how materials perform under these challenging multi-environment conditions.
Prohesion Test Method
Another notable Japanese development is the Prohesion Test method.
This method focuses on cyclic testing by spraying corrosive solutions and repeatedly drying out the specimens. The Prohesion Test aims to simulate environmental conditions where corrosive substances are deposited on surfaces intermittently and then exposed to humidity or moisture fluctuations.
Advantages of Prohesion Testing
This cyclic approach allows for sped-up evaluation of material durability and identification of potential weaknesses that may arise from repeated wet-dry cycles.
Introduction of the Scab Test
Japan has introduced the Scab Test as an alternative CCT method.
In this test, a dry abrasive material is applied to samples before being subjected to salt spray exposure. The abrasion aims to mimic mechanical damage in real-life scenarios when surfaces come into contact with sharp objects or rough handling occurs during transportation or installation.
Impact of Japanese CCT Methods
These Japanese-developed CCT methods have played a crucial role in expanding our understanding of material performance under diverse environmental conditions that encompass chemical aggressiveness and mechanical stressors.
They have provided researchers worldwide with effective tools for assessing the durability and corrosion resistance of various materials used across industries, from automotive manufacturing to electronics.
Global Influence and Future Research
It should be noted that these contributions from Japan have not only expanded salt spray corrosion testing but have also spurred further research and development efforts globally.
As the demand for materials with improved corrosion resistance continues to grow, the Japanese advancements in CCT methods serve as a foundation for innovation and future improvements in corrosion testing techniques.
The never-ending quest to improve salt spray corrosion testing reflects our determined spirit of triumph over corrosion. While this cunning foe continues trying to claim its metallic prey, humankind steadfastly counters with enhanced tools and techniques.
Our greatest counteroffensive is the power of science itself – through meticulous research, shared knowledge, and a commitment to progress. We inch ever closer to materials that can endure corrosion’s ceaseless attacks.
As we stand at the frontier of this scientific battlefield, we know new challenges await. But with continued teamwork and vision, we ensure corrosion meets its match and our creations last for generations.