Application Study of Pickling Corrosion Inhibitors for Carbon Steel

Abstract

This application study systematically compares thecorrosion inhibition performance of the carbon steel pickling corrosion inhibitor Vanconol®IR-90 against competing productson different grades of carbon steel substrates in various acid solutions,aiming toprovide data support for engineering material selection.

1 Introduction

In industrial cleaning, pickling is a core process that utilizes acidic solutions to remove oxide layers, rust, and contaminants from metal surfaces, and is widely applied in metal surface pretreatment fields such as metal processing and equipment maintenance. However, while the acid solution dissolves the scale layer, it also causes corrosive damage to the base metal, leading to significant economic losses from equipment repair, production shutdowns, and product scrap. Therefore, selecting an appropriate pickling corrosion inhibitorhas become a key approach to balancing pickling efficiency with the safety of the metal substrate.

Common pickling systems include: inorganic acid solutions, such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and hydrofluoric acid; and organic acid solutionssuch assulfamic acid,citric acid,tartaric acid, glycolic acid,oxalic acid, and chelating agent-type organic acids. Different acids exhibit varying corrosion (dissolution) capabilities on metals, and their suitability for metal substrates during cleaning also differs accordingly.The suitability of common pickling solutions for metal substrates is shown inAppendix Table 1.

Carbon steel, as a major category of metal material in industrial cleaning, requires the development of highly effective corrosion inhibitor products for the protection of its substrate.This application study systematically compares thecorrosion inhibition performance of the carbon steel pickling corrosion inhibitor productVanconol®IR-90 againstcompeting productson different grades of carbon steel substrates in various acid solutions, aiming to provide data support for engineering material selection.

Experimental Section

Experimental Materials

Metal substrates:#20 carbon steel,#45 steel;

Corrosion inhibitor: Vanconol®IR-902 and a competitor product (typical parameters of each product are listed in Table2);

Table2 Parameters of corrosion inhibitors used in the test

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Item	Vanconol®IR-902	Competitor product
Appearance	Reddish-brown uniform transparent liquid	Brownish-yellow transparent liquid
Density (20℃)	0.95~1.05g/cm³	1.06~1.08 g/cm³
Solid content	32%~36%
pH value	3.0-4.0	≥7
Solubility	Readily soluble in water/alcohol, miscible with acid solutions	Readily soluble in acid solutions
Temperature resistance	≤90℃	≤90℃
Compatible Acid Media	Suitable for pickling with various inorganic acids, organic acids, and mixed acids	Suitable for pickling with various inorganic acids, organic acids, and mixed acids

Inorganic acid pickling systems:10% HCl solution,10% H₂SO₄ solution,10% H₃PO₄ solution,10% sulfamic acid solution;

Organic acid pickling systems:3% citric acid solution,3% ammonium citrate solution (pH=3~4),10% EDTA solution.

Evaluation Method

Solution preparation: First, add the weighed amount of corrosion inhibitor to the weighed amount of water, and stir thoroughly to prepare a0.3% aqueous solution. Then, slowly add the weighed amount of acid solution to the above inhibitor solution and stir thoroughly.

Prepare test coupons and treat them in accordance withHG/T3523. Coupon types include20# carbon steel,N80 steel, andXT45 steel coupons.

Perform static coupon immersion tests to evaluate corrosion behavior.

Evaluation Criteria

Corrosion Rate

Where:

V —corrosion rate, in mm/a;

m₀ — initial mass of the specimen before the test, in g;

m₁ — mass of the specimen after the corrosion test, in g;

87600 — calculation constant;

S — surface area of the specimen, in cm²;

ρ — density of the specimen material, in g/cm³;

t — test duration, in h.

Calculation of corrosion inhibition efficiency

where:

V₀ — corrosion rate without the corrosion inhibitor, in mm/a;

V₁ — corrosion rate with the corrosion inhibitor, in mm/a.

Evaluation criteria

The rating criteria for the substrate corrosion test are presented in Table 2 below.

Table 2 Rating criteria for metal corrosion tests

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Metal Substrate	Grade	Coupon Specimen Condition
		Appearance	Mass Change (Unit: mg)
Steel and Cast Iron	Grade 0	No corrosion pits on surface, no obvious discoloration	<2
	Grade 1	No corrosion pits on surface, slight discoloration or loss of luster	<2
	Grade 2	Obvious discoloration and loss of luster on surface	≥2
	Grade 3	Visible corrosion pits on surface	≥2

Note: Excerpted from JB/T4323-2019, “Water-Based Metal Cleaners” standard.

Results and Analysis

Corrosion Results Analysis of 20# Carbon Steel

Fig.1 Comparison of corrosion rates of 20# carbon steel in different acid solutions

Fig.2 Comparison of corrosion inhibition efficiencies of inhibitors on 20# carbon steel in different acid solutions

20# carbon steel is a low-carbon steel with good toughness, plasticity, and weldability. It can be used for welded structural parts, pipe fittings, flanges, light-duty gears/sprockets, bolts, and low-load shafts. As shown in Figs.1 and2, the Vanconol®IR-902 corrosion inhibitor exhibits superior corrosion control performance over the competing product in all acid solutions tested. In each pickling solution, the corrosion rate remained below1 g/(m².h), with corrosion inhibition efficiencies all exceeding 99%. The substrate surface was rated as Grade 0. This indicates that the inhibitor possesses broad-spectrum applicability and can be used to control uniform corrosion of this type of carbon steel while improving cleaning efficiency.

Corrosion Results Analysis of 45# Steel

Fig. 3 Comparison of corrosion rates of 45 carbon steel in different acid solutions

Fig. 4 Comparison of corrosion inhibition efficiency of the inhibitor on 45 carbon steel in different acid solutions

45 carbon steel is a medium-carbon steel with high strength and hardness, and can be used for load-bearing components such as crankshafts, main shafts, gears, connecting rods, and piston pins. As shown in Figs. 3 and 4, Vanconol®IR-902 exhibits superior corrosion inhibition performance in all tested scenarios except for a slightly weaker effect in hydrochloric acid pickling compared to the competitive product, achieving an overall corrosion inhibition efficiency of >99%. The substrate surface was rated Grade 0, indicating that the inhibitor effectively reduces the risk of hydrogen embrittlement or over-pickling, thereby protecting the substrate.

Conclusions

Vanconol®IR-902 corrosion inhibitor can effectively control corrosion and improve pickling efficiency. The product features the following characteristics:

Broad-spectrum performance: applicable to various organic and inorganic acid pickling scenarios with a wider processing window;

Applicable scenarios: suitable for pickling and rust removal, industrial equipment recirculation cleaning, pre-commissioning cleaning, short-term immersion cleaning of machined parts, mild cleaning of high-precision industrial parts with organic acids, and weld slag removal from workpieces.

Appendix

Appendix Table 1 Adaptability of common acid solutions to metal substrates

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Acid Type	Applicable Materials	Key Application Scenarios	Characteristics
Hydrochloric Acid (HCl)	Carbon steel, copper and copper alloys	Descaling and mill scale removal for carbon steel, copper and copper alloys; cleaning of boilers and heat exchangers dominated by carbonate scale.	Strong dissolving power: rapid dissolution of iron oxide and calcium carbonate scale. Effective results: bright workpiece surface after pickling with minimal residue. Low cost: inexpensive, making it the“economical choice” for pickling. High volatility: prone to acid mist generation, causing environmental pollution and equipment corrosion. Material restrictions: strictly prohibited for stainless steel (prone to pitting and stress corrosion) and aluminum.
Sulfuric Acid (H₂SO₄)	Carbon steel, stainless steel, aluminum alloys	Pickling of carbon steel, stainless steel, and aluminum alloys; removal of difficult-to-dissolve scale containing silicates.	Wide applicability: suitable for stainless steel and aluminum where hydrochloric acid is not applicable. Low cost: distinct cost advantages. Non-volatile: convenient for heating operations without acid mist concerns. Slow reaction: poor effectiveness at ambient temperature; typically requires heating to50-60°C. Scale-prone: may generate secondary precipitates such as sparingly soluble calcium sulfate.
Nitric Acid (HNO₃)	Stainless steel, aluminum and aluminum alloys	Pickling of stainless steel, aluminum and aluminum alloys; used as an oxidizing acid for passivation treatment.	Non-damaging to stainless steel: no corrosive embrittlement effect on stainless steel, making it an excellent substitute for hydrochloric acid. [[CONTENT]]
Nitric Acid (HNO₃)	Stainless steel, aluminum alloys, carbon steel (limited)	Passivation treatment of stainless steel; descaling of equipment with stringent cleanliness requirements.	Strong oxidizing power: effectively dissolves a wide range of metal oxides and organic matter. Unstable: readily decomposes upon exposure to light and heat, generating toxic nitrogen oxide gases. Extremely rapid corrosion of carbon steel.
Hydrofluoric Acid (HF)	Stainless steel, castings (requires strict control)	Highly effective for removing silica scale; pickling of stainless steel and castings (often used in mixtures with other acids).	Specific action on silica: one of the few acids capable of effectively dissolving silicate scale. Rapid dissolution: high efficiency in dissolving iron oxide scale. Highly toxic: poses extreme hazards to human health and the environment, requiring very stringent handling procedures. Highly corrosive: exhibits strong corrosive attack on high-alloy steels.
Phosphoric Acid (H₃PO₄)	Carbon steel, stainless steel, copper alloys, and most other metals	Pickling of precision parts and machined components; applications with stringent corrosion requirements.	Mild: relatively weak acidity, less prone to causing over-etching and hydrogen embrittlement. Forms protective film: leaves a phosphate conversion coating on the surface after cleaning, which helps prevent flash rusting. Slow action: rust removal efficiency is significantly lower than that of strong inorganic acids. Higher cost: relatively expensive.
Sulfamic Acid (NH₂SO₃H)	Stainless steel, copper and copper alloys, carbon steel	Descaling and rust removal for stainless steel, copper, and their alloys where chloride ions are a concern; industries with high cleanliness requirements such as food and electronics; boiler and heat exchanger cleaning.	Solid form, easy to store and transport, non-volatile, providing a better working environment. Chloride-free, posing no risk of stress corrosion cracking (SCC) to stainless steel; a safe alternative to hydrochloric acid. Mild reaction, low corrosivity to the base metal of equipment, and less prone to inducing hydrogen embrittlement. Easy disposal of spent cleaning solution, offering good environmental compatibility. Weak ability to remove rust; less effective against iron oxide scale. Higher price compared to hydrochloric acid and sulfuric acid. Decomposes readily above 60°C, leading to loss of effectiveness.
Citric acid (C₆H₈O₇)	Stainless steel, carbon steel, copper alloys	Pickling of stainless steel and precision components; removal ofcalcium and magnesium scale; cleaning of food and pharmaceutical equipment.	Non-toxic and safe, with good biodegradability and favorable environmental and operator compatibility. Low corrosivity to stainless steel, serving as a green alternative to hydrochloric acid. Through chelation, it effectively dissolves and complexes metal ions, preventing re-precipitation. Relatively weak acidity results in lower cleaning efficiency, typically requiring heating or extended soaking time. Reaction with calcium ions may produce calcium citrate precipitate, requiring control of concentration andpH. Relatively higher cost.
EDTA (ethylenediaminetetraacetic acid)	Applicable to almost all metal substrates as an auxiliary additive	Chelating agent, not a primary acid. Used for treating complex scales containing sparingly soluble calcium sulfate and iron deposits; commonly employed as an additive in hydrochloric acid and citric acid cleaning solutions.	Exceptionally strong chelating capability that can “capture” and dissolve metal ions (e.g.,Ca²⁺, Fe³⁺) that conventional acids cannot handle, preventing re-precipitation. Significantly enhances the cleaning efficiency and application range of primary acids. Very weak acidity by itself; cannot serve as the sole pickling agent. High cost, typically used only as an auxiliary additive. Requires pH control and decomposes at elevated temperatures.
Ammonium citrate	Carbon steel, stainless steel, copper alloys, etc.	Petroleum pipeline cleaning; complexing agent in electroplating baths; used as a buffering and complexing agent in cleaning formulations.	Cleaning capability is not its primary advantage; it serves more as a functional additive. Limited application as a primary cleaning agent in mainstream industrial pickling operations.

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