Corrosion-resistant alloy impellers are widely used in corrosive environments such as chemical processes, seawater pipelines, sewage treatment facilities, and high-salt and humid environments. They are an essential component ensuring equipment operating reliability and service life.
This article discusses surface passivation treatment procedures required for typical corrosion-resistant alloys such as austenitic stainless steel and nickel alloys before they are put into service, explores the principle of forming dense passivation films to impart pitting resistance by chlorides and crevice corrosion resistance, and establishes corrosion grade and morphology of passivation-treated impeller specimens with different passivation methods through standardized neutral salt spray tests (NSS). The experiments show that electrochemical passivation greatly enhances surface stability and film compactness, especially in offering enhanced protection for nickel alloys such as Inconel 625. The paper provides the theoretical basis and process design for optimization of corrosion life prediction and surface treatment of corrosion-resistant alloy impellers.
Introduction
In their long-term use in very corrosive media (such as acids, alkalis, high-chlorine mediums, seawater, or industrial wastewater containing SO₄²⁻), impellers due to their high-speed rotation and wetted to liquids are the most prone pieces of equipment to be damaged by corrosion. Even when high-corrosion-resistant materials such as 316L, duplex stainless steel, Hastelloy, and Inconel 625 are used, their surfaces become degraded with passivation film through casting, processing, or welding, resulting in exposed active metal sites that provide a starting point for corrosion attack. Therefore, post-treatment for rejuvenating and enhancing the protective behavior of the passivation film is the most significant technique of how to provide long-life and low-maintenance operation, particularly to delay pitting and crevice corrosion in chloride-bearing solutions.
Passivation treatment, as one of the widest used surface modification technologies, not only has the ability to increase the chemical corrosion resistance of the impeller surface, but also its surface condition, building a solid foundation for subsequent coating and sealant application. In order to evaluate its performance, standard salt spray tests and other environmental simulation methods rely on to quantify its protective performance and endurance.
Principles and Technical Paths of Passivation Treatment
In multi-complex service conditions corrosion countermeasures, passivation surface treatment, as a non-sacrificial, green, and efficient metal protection technology, has outstanding protective performance. Compared with anodic plating or coating materials, passivation treatment, due to nanoscale thickness, high compactness, and excellent self-healing ability, can effectively seal off corrosive media for a long time, and is therefore particularly fit for high-performance metal materials such as stainless steel, nickel-based alloys, and titanium alloys. In large components such as impellers, heat exchange tubes, and fasteners, passivation not only increases corrosion resistance but also excludes interference by electrochemical corrosion side reactions on the overall structure, thereby making it an integral part of building high-reliability components. The following discusses from basic principles of passivation, common process types, to individual processes for general materials.
Electrochemical Nature of Passivation
Passivation is to form a self-healing, compact protective film (mostly Cr₂O₃, NiO, MoO₃, etc.) of a few nanometers to tens of nanometers in thickness on the metal surface by chemical or electrochemical means. The film as a “barrier layer” between metal and the ambient corrosive media might significantly reduce the anodic dissolution rate of the metal and cut off the electrochemical reaction chain in the corrosion process, thereby inducing “surface inertization”.
In my opinion, the core of passivation lies in its “non-sacrificial” protection mechanism. Unlike typical anodic plating (e.g., galvanizing), the passivation film itself is not particularly reactive, but protects the substrate through “barrier” and “passive stability”. In the case of high-concentration chloride ion environments in particular, only a structurally stable and self-healing passivation film can successfully resist pitting penetration over a long period of time.
Types of Passivation Treatment Processes
| Passivation Type | Process Description | Applicable Materials |
| Chemical Passivation | Treatment with solutions such as nitric acid or citric acid to form a surface oxide film | Stainless steel, duplex stainless steel |
| Electrochemical Passivation | Promoting oxide film formation through electrolysis, resulting in a uniform and dense film | Nickel-based alloys, titanium alloys, etc. |
| Environmentally Friendly Alternative Passivation | Using environmentally friendly passivators such as citric acid/phosphoric acid instead of traditional toxic chromic acid | Austenitic stainless steel, environmental protection scenarios |
Different materials are to be combined with different passivation treatments. For example, 316L stainless steel is suitable for chemical passivation using citric acid or dilute nitric acid, while high-Ni alloys such as Inconel 625 are more suitable to electrochemical passivation to preserve film integrity and compactness.
Process Flow Control (Taking 304 Stainless Steel as an Example)
Pretreatment: Degreasing → Pickling (10% HNO₃ + 2% HF to remove oxide scale);
Passivation: Immersion in 20–30% HNO₃ or 8–10% citric acid for 30–60 minutes;
Post-treatment: Rinse with deionized water → Dry → Sealing treatment (such as coating);
According to my experimental experience, pretreatment quality is a very significant factor in the passivation effect at the final stage. Oils or oxides adhered to the surface will have a serious effect on the deposition of the passivation film, leading to non-uniform film layers or even local corroded spots in the next stage. Therefore, pretreatment and liquid parameter control (pH, temperature, metal ion concentration) are of equal importance.
Principles and Implementation of Salt Spray Test
In aviation, marine, chemical, and new energy equipment with high corrosion risks of use are applications where corrosion resistance of the metal material and protective structure of surface is an essential factor that decides the service life and reliability of the critical components. As a quick test method to simulate corrosion behavior in sea atmospheric environments, the salt spray test is widely used in the research and development of materials, surface treatment quality inspection, and quality testing due to its high reproducibility, simple operation, and short cycle.
Especially for components such as impellers, fasteners, and heat exchanger components, the salt spray test has been the preferred method to measure coating integrity, passivation film stability, and plating continuity. By comparing the corrosion behavior of the different process routes or material systems under simulated corrosive conditions, engineers can quickly screen out the most effective protection solution(s) and thus eliminate successfully corrosion failure risks in future service.
Test Purpose and Significance
The salt spray test essentially simulates the corrosive process of the marine atmospheric environment by spraying a 5% sodium chloride solution into an enclosed test chamber under controlled temperature, humidity, and spray rate conditions. Its intrinsic purpose is to induce corrosion reaction of materials or coatings within a relatively limited period of time (usually 72 to 168 hours), in an attempt to evaluate their corrosion resistance in terms of different aspects including macroscopic appearance change, mass loss, and surface morphology evolution.
This speedup aging effect is particularly suitable for the early screening of the protective performances of surface treatment technologies such as passivation films, electroplating coatings, and thermal spray coatings, whose test cycle and cost for long-term natural exposure tests can be greatly reduced. Meanwhile, for quality consistency verification of standardized production, salt spray test can also provide a good supporting role for process stability.
Test Conditions and Technical Standards
Salt spray test reliability and reproducibility necessitate rigorous test conditions and utilization of international standards. Most commonly used salt spray medium is a 5% NaCl solution of water filtered before spraying in order to remove impurities. The temperature for testing is constantly controlled at 35 ± 2 ℃ to ensure stability of solution atomization efficiency and corrosion reaction rate stability, and the pH value is regulated between 6.5-7.2 to ensure the neutral character of the solution, replicating the common corrosion behaviors of seawater media.
The spraying technology employs continuous gravity spraying or pressure atomization, with an atomized particle diameter of 12 mL/h·80cm². Typical specifications are ASTM B117 in the USA, ISO 9227 in the EU, and GB/T 10125 in China, with explicit specifications on equipment of tests, sample position, monitoring method, and test evaluation cycles. Depending on relative application scenarios, test time can be from a minimum of 24 hours to more than 1000 hours, adaptable to accommodate the assessing requirement of different corrosion resistances.
Post-Test Evaluation Dimensions
After the salt spray test, carry out a multi-dimensional evaluation method by requiring systematic analysis of the corrosion extent and protective performance. Determine the corrosion grade by first classifying it according to the 0~10 grading scale of ASTM D610 standard in terms of corrosion area ratio and pitting morphology to intuitively determine the sample’s extent of surface failure. Second, morphological analysis can track micro-damage characteristics such as surface corrosion pit geometries, film detachment, and crack direction developments via a scanning electron microscope (SEM) to reveal the mechanism of corrosion and failure initiations.
Thirdly, component analysis identifies the types of corrosion products and distribution of elements by methods such as X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) to determine the chemical stability of the protective film or passivation film under the salt spray condition. Fourthly, the rate of mass loss is a measure of the rate of corrosion calculated by means of the difference in mass difference of the sample before and after test, widely used for representing the quantitative data of overall corrosion capability of the material and for judging corrosion uniformity and quality prediction. These testing methods are complementary, building a system of corrosion testing from macro to micro, from appearance to function, based on a scientific foundation for subsequent material selection and surface optimization.
Comparative Analysis of Passivation and Salt Spray Test
As examples, for 316L stainless steel and Inconel 625, the authors conducted a 168-hour neutral salt spray test after three types of treatments. Some of the experimental results are as follows:
| Material Type | Passivation Method | Salt Spray Grade | Pitting Condition |
| 316L | Unpassivated | IV | A large number of black corrosion spots |
| 316L | Citric acid passivation | II | Slight pitting |
| 316L | Electrochemical passivation | I | No obvious corrosion traces |
| 625 alloy | Unpassivated | III | Obvious pitting and pits exist |
| 625 alloy | Electrochemical passivation | I | The surface is intact without corrosion |
It can be seen from the table very easily that passivation, especially electrochemical passivation treatment, has evident advantages in increasing general corrosion resistance and chloride pitting resistance, and can be employed to greatly enhance the service life of corrosion-resistant alloy impellers in harsh working conditions.
Conclusion
The service performance of corrosion-resistant alloy impellers not only depends on the material itself but also their surface condition and protective treatment level. Reasonable choice and optimization of passivation treatment, especially electrochemical treatment, can effectively improve compactness and stability of surface passivation film and greatly improve corrosion resistance of material in salt spray media. Combination of salt spray test with microanalysis can provide enough evidence to impeller service safety assessment and corrosion life estimation, and ensure long-term stable operation of industrial equipment.
