Flux Quality: Concentration, Density (Baume), Flux Ratio, pH
What are Baumé, Flux Ratio, and pH, and how do these affect flux quality?
The purpose of using a zinc-ammonium chloride flux solution in the pretreatment process for hot-dip galvanizing is to clean the steel of all oxidation developed since the pickling stage and to create a protective coating to prevent further oxidation before entering the galvanizing kettle. Whether using a preflux tank, wet kettle, or a combined fluxing process in your galvanizing plant, it is critical to understand the general properties of the bath solution which can greatly affect the quality of the flux and therefore the quality of the hot-dip galvanized coating.
For optimum performance, concentration, flux ratio, and pH of the flux solution should be controlled, along with contaminants. Regular monitoring of these bath properties is helpful when scheduling required maintenance activities and/or for troubleshooting problems related to coating quality. Below are descriptions of the primary bath properties and their relevance to flux performance:
Concentration and Density (in Degrees Baumé)
The concentration of a flux solution is defined as the ratio of flux salts to water in the solution. The recommended concentration depends on the humidity of the air, the time delay between fluxing and galvanizing, the type of operation (preflux vs. top flux), flux ratio, and the level of oxides on the work. To monitor the concentration, the density of the flux solution is measured in degrees Baumé using a hydrometer, and the flux supplier should be consulted for specifics on the recommended density of the flux.
If the Baumé value of a preflux solution is too low, rapid formation of red rust can occur after drying, resulting in a poor galvanized coating affected with bare spots. A high Baumé value should be considered for plants in high-humidity or corrosive environments, or when cleaning and pickling leave heavier oxide layers. Higher concentrations are also useful in combined preflux and top flux operations, as the preflux carry-over results in self-maintenance through small, continuous additions to the top flux blanket.
The flux ratio refers to the balance by weight of zinc chloride to ammonium chloride in the flux solution, and the recommended ratio depends on the same factors as concentration. The flux ratio can be relevant when addressing issues related to coating quality because the ability of the flux solution to remove oxides from the steel surface is controlled by the amount of ammonium chloride. Therefore, higher levels of ammonium chloride result in a lower flux ratio and a more active flux solution. The two most common ratio fluxes are at 0.85 and 1.27. These ratios are known as triple salt and double salt, respectively, due to the chemical formulations of the fluxes.
A low flux ratio is useful in facilitating the coating of work that is difficult to galvanize, or when cleaning or pickling is less than perfect. A low flux ratio will result in parts that dry faster but will have less protection against reoxidation of the steel during the time between pre-fluxing and galvanizing. Furthermore, flux solutions with a low flux ratio can lead to higher fume emissions and skimmings output.
A high flux ratio will provide better protection of the work against reoxidation between pre-fluxing and galvanizing but are not as tolerant of poor cleaning and pickling practices. Reduced emissions of smoke and skimmings can also be expected when using a high flux ratio, especially if using a top flux.
Generally, pH is a measure of the hydrogen ion concentration of a solution. Flux solutions with a high concentration of hydrogen ions have a low pH (and are acidic) and solutions with low concentrations of hydrogen ions have a high pH (and are basic). After hydrochloric or sulfuric acid pickling, some acid and dissolved iron are carried over into the flux solution and affect the pH value. Variation in pH can also impact the efficiency of flux filtration systems. Furthermore, it is optimal to adjust and maintain pH of the bath for easier removal of impurities such as iron and sulfates.
If the flux solution becomes too acidic (low pH), the ability of the preflux solution to provide oxidation protection for the steel products is reduced and may lead to uncoated areas (bare spots) during galvanizing. A low pH will also increase corrosion of heating coils and increase the amount of soluble iron in the flux bath, leading to increased dross formation in the galvanizing kettle. Additionally, if the pickle acid is sulfuric, the flux bath can also become contaminated with sulfates which can interfere with proper flux activity.
If the flux solution becomes too basic (high pH), decomposition of zinc chloride in the preflux solution will occur, decreasing the effectiveness and preventing adequate drying of the steel after removal from the preflux tank.
AGA Resources on Flux Solutions
For more information on concentration, density in degrees Baumé, flux ratio, and pH, the American Galvanizers Association (AGA) has the following resources available for information on recommended values and maintenance best practices:
Galvanizing Note: Fluxing Troubleshooting Guideline: Measuring & Adjusting pH of the Preflux Solution AGA Process Seminars: #2 Galvanizing Chemistries and Solutions AGA Processing Video: Processes in The Galvanizing Plant DVD
© 2020 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.