Temporary rust preventives—A retrospective

Progress in Organic Coatings 140 (2020) 105511

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Review

Temporary rust preventives—A retrospective Viswanathan S. Saji

T

Center of Research Excellence in Corrosion, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, 31261, Saudi Arabia

ARTICLE INFO

ABSTRACT

Keywords: Corrosion Temporary rust preventives Rust preventive oils Electrochemical studies

Temporary rust preventives (RPs) are essential in several industrial applications in protecting the metallic structures from corrosion during manufacture, storage and transportation. This review provides a comprehensive account of research advancements in temporary RPs. The review starts with a brief introduction to RPs and their various types. § 2 gives an account of old reported works as several interesting works in this area appeared before the 1980s. § 3 explains the recent R & D on RPs where a separate section on patents are added as most of the latest reports are patented information. § 4 discusses different electrochemical methods employed in studying RPs. The review ends with a short description of commercial RPs. Future scopes of R & D are also presented.

1. Introduction Rust formation on metallic structures is a spontaneous process [1–4]. The rusting during manufacturing, storage and transportation of equipment and components is a critical issue, especially under humid and saline environments. Temporary rust preventives (RPs) are often employed in these cases as a rust inhibiting barrier coating. RPs isolate the metal from aggressive environments (moisture, salts, acids, etc.) typically to avoid atmospheric corrosion. RPs are also known by other names such as temporary rust preventive coatings (RPCs), and rust preventive oils (RPOs, where oil become a part of the composition). They find applications in all metallic surfaces where a painting is not a feasible approach. The application areas include heavy machinery, military equipment, vehicle undercarriage, packaging, offshore drilling equipment, boat shafts and propellers, marine fittings, construction materials, turbine blades, generators, steel fasteners, ball and roller bearings, steel pipes and tubes etc. [5–10]. A market survey conducted in 2012 showed that around 300,000 tons of metal-protecting fluids were used around the world which was ∼ 12 % of the total market of metal-working fluids. Asia is the largest consumer of RPs, with China leading the scenario. Solvent-based or oil-based RPs dominate the Asian and American markets, whereas European markets practice a greater percentage (∼ 40 %) of environmentally friendly water-based RPs [8,11]. Several companies worldwide are involved in the business of RPs; to mention a few, Zerust Excor, Bechem, Wedolit, Hindustan Petroleum, Fuchs, Zavenir Daubert, Cortec, and OKS. There are several types of RPs. A classification is shown in Fig. 1. They are usually made by mixing rust inhibitors, film-forming agents and other functional additives into a primary carrier (base oil, solvent,

water, etc.) [8,12]. Tate and Beale classified RPs into soft film, hard film, and oil film. The soft film was subdivided into solvent-deposited thin film, hot-dip thick film, smearing and slushing types [6]. Vapor phase corrosion inhibitors (VCIs) and contact inhibitors are considered as a type of temporary RPs. Resin/plastic-based strippable coatings are another class. The choice of a proper RP depends upon the metal to be protected and the period of protection required, and the existing environmental conditions [6–8,13]. Corrosion inhibitors constitute a major component in industrial RPs where they enhance the barrier protection of RPCs. Several important works in this area reported before the 1980s (see § 2). The major inhibitors employed are adsorption-type organic compounds having one or more polar groups (with oxygen, nitrogen, phosphorus, sulfur atoms, and π electrons) with water-repellent properties. Sulfonate, carboxylate, and ester-type inhibitors are normally employed. Inorganic inhibitors such as nitrite, silicate, phosphate etc. have also been used in industrial RPs especially in water-based formulations [14,15]. RPs can be applied by immersion, brushing, roller, swabbing, wiping, spraying (conventional or electrostatic), dipping (cold and hot), circulation, or drip methods. The application technique often relies on the size, shape and nature of the material to be protected [5,8,13]. An industrial RP is expected to have excellent antirust effect with good film-forming character and mechanical properties and is expected to be suitable for the different coating methods. It should be stable, oxidation resistant, and reasonably inert. It should not affect the underlying metal and should be easy to apply and remove. The RP needs to be acceptable in terms of their level of biodegradability, bioaccumulation and toxicity according to international standards. Easy removal of the RP is essential to the better performance of succeeding processing steps like painting,

E-mail address: [email protected]. https://doi.org/10.1016/j.porgcoat.2019.105511 Received 15 October 2019; Received in revised form 23 November 2019; Accepted 16 December 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.

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low fire hazard, and easy removal. Vegetable oils and their esters are having several properties similar to their petroleum-based equivalents and are part of several formulations. VCIs can be employed together with RPOs [18]. However, the RPOs may wipe off during handling also they may not withstand severe conditions. Due to the oil content, the coated part will never get dried and remains oily in appearance [6–8,18]. 1.1.3. Water-based Replacing solvent and oil-based carriers with water-based systems is an eco-friendly approach. Currently, anticorrosion emulsions are preferred in several applications [34]. Different from RPOs, the performance of an anticorrosion emulsion is prejudiced not only by the nature of corrosion inhibitor but also by the type of emulsifier incorporated. In emulsified form, the films typically provide only short-term protection; but are non-flammable. On the other hand, drying times are slow, and the impurities can be a cause of problems. Water-based RPs do not efficiently displace water, so surplus pre-treatment/cleaning may be required before application [8]. They can be applied successfully in some but not all areas of use. There exists increasing interest in developing novel water-soluble corrosion inhibitors with higher rust preventive performance [12].

Fig. 1. A classification of temporary rust preventives.

phosphating, galvanizing, or welding. RPs, in general, can be removed by solvent cleaning, wiping, and aqueous alkaline, neutral or acidic cleaners [7,8,13,16–18]. RPs in specific compositions may perform additional functions of lubricants [5,7]. Several reports are available on developing RP compositions with high lubricity [19–21]. The following section (§ 1.1) provides concise descriptions of different types of RPs. Most of the reviews or articles on RPs were published before the 1980s [5,22–33]. A few reviews addressed polar inhibitors and their action mechanism in RPOs [32,33]. Several old reviews are available in other languages, especially Japanese. There are no recent comprehensive reviews available on the topic except an article based on a webinar in 2014 by McGuire [8] and a book chapter in 2010 by Tate and Beale [6]. Both the references, however, are written in an industrial perspective with less emphasis on research advancements in this area. It is essential to develop novel high-performance RPs with optimized properties and greater rust prevention. Currently, researchers are paying attention to fabricate modified and novel RPs with improved performance coupled with environment-friendliness at a reduced cost. Literature analysis showed that plenty of topical information is available on this topic, and a significant fraction of it is patents. The present review aims to provide a comprehensive description of the research advancements in various RPs. A section on electrochemical studies in evaluating RPOs is also provided.

1.1.4. Wax-type The grease and wax-like protectives are preferred when customers are seeking a soft heavy build film. The typical semisolid and solid formulation can contain oil component, wax component, water-displacing and rust-preventive agents [26]. Molten wax films can ingress into inaccessible component surfaces such as in engine compartments. Petroleum waxes (paraffin wax, petrolatum, and microcrystalline waxes) are a favoured technology for years. The most widely used paraffin waxes can be further classified as slack wax (wax that contains oil content as high as 35 %) or scale wax (having an oil content of 1–2.5 % [13,35]. 1.1.5. Resin-type These are resin or plastic-type coatings, usually of a heavy build (applied by hot or cold dip), and can provide long-term protection. They are removable by mechanical stripping or by alkaline/solvent washing. The most important of resin-type coatings are polymer resinsbased with comparatively high dipping temperatures (∼ 200 °C). The films deposited in the cold are typically robust and neither sticky nor brittle. The solvents used differ depending on the ingredient’s solubility, drying-time, flammability, and permitted toxicity [6].

1.1. Types of RPs 1.1.1. Solvent-based These are the coatings deposited on surfaces by the evaporation of solvents (hard or soft film with quick-drying or slow-drying). Usually, they are made from low viscous formulations with incorporated rust inhibitors, waxes and various additives. As the solvent evaporate quickly, the resulting film generally appears much drier than those based on oils. The period of protection may range from a few days to over a year. They are easy for application and are suitable for severe corrosive conditions. They can be removed easily using conventional industrial cleaning techniques [6–8]. The solvent-based RPs are however not attractive in terms of green guidelines as they can cause health, safety and ecological problems. Their low-flash point is also a concern. Surplus time may requisite for solvent evaporation, and the removal step may become challenging for a hard film [8,26].

1.1.6. Volatile corrosion inhibitors (VCIs) VCI technology offers an important alternative to RPOs [36,37]. VCI typically have vapor pressures at the range of 10−3 to 10−7 mm Hg to allow vaporization and subsequent adsorption and formation of a thin film on metal surfaces. They can reach and shield even the unreachable parts of the metal to be protected. Various types of amines such as cyclohexylamines, aminonitrobenzoates, heteroalkylated amines are mainly used as VCIs. They are employed in different forms such as films, papers, porous emitters (desiccants) etc. [38]. The method is attractive as no oily components are involved, and the removal process is relatively easy. Gangopadhyay and Mahanwar classified the VCI coatings into strippable, permanent, water-based, solvent-based and green coatings [38]. Several recent reports are available that utilize synergism between different RPs with VCIs. However, VCIs may harm nonmetallic objects such as plastics. Many VCIs are environmentally unfriendly too [6,16]. As opposed to contact inhibitors, VCIs have a sufficient vapor pressure to be volatile, saturate the vapor phase and create a self-organizing protective layer on the metal surface [39]. More details on VCIs can be found elsewhere [36–45]. There are various other temporary RPs such as desiccators, dehumidifiers, etc. More details of different RPs can be found elsewhere

1.1.2. Oil-based Oil-based RPs (RPOs) can be suitable for rust protection as well as lubrication. Their volatile organic content (VOC) is lesser than that of solvent-based systems and hence are more environment-friendly. The base oil is usually selected from mineral or petroleum oils. Besides the oil component, traditional RPOs use paraffins, oil-soluble corrosion inhibitors, antioxidants, dewatering agents and various speciality additives. The viscosity can range from ∼ 4 cSt at 40 °C, to highly viscous fluids. The advantages include easy application, abrasion resistance, 2

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[6–8]. The next section provides a comprehensive description of reported works on RPs before 1985.

to a mixture of calcium or barium mahogany sulfonate in a crankcase oil [74]. Fatty acids and waxes are standard components in RPO formulations. Rudel and Morway reported that RPOs for hot-dip applications could be thickened and gelled by a synergistic mixture of microcrystalline wax (I) and polyethylene (II). To a mineral oil (viscosity 25–500 SSU, 210 °F, pour point < 30 °F), 1−10 wt.% of gelling agent (mixture of I and II at the desired composition), and 1–10 % of rust inhibitors (metal sulfonates and the fatty acid partial esters of aliphatic polyhydric alcohols) were added [75]. Stickdorn reported a composition of fatty amine, complex-forming metal, and soap-forming carboxylic acid (natural or synthetic fatty acid or naphthenic acid). The homogeneous viscous substance obtained was diluted with benzene, CCl4, mineral oils or fats (1–30 %) to obtain oils, greases, or suspensionbased RPs depending on the application [76]. Marumo and Maruyama reported a series of RPs based on oil fatty acids. The authors showed that when a rust preventive substance was contaminated with inorganic salts in an amount more than that soluble in the oil, the inorganic salts accelerate rusting [77,78]. In a typical patented RP composition, the reaction product of lauryl amine with a large excess quantity of methyl acrylate was saponified with NaOH, and the soap was treated with CaCl2 to give Ca β,β'-(N-laurylimino)-dipropionate which was dissolved in 97 g spindle oil [77]. Dodon reported a coating composition containing asphaltic or heavy naphthenic mineral oil (viscosity 2.3−4 °E at 100 °C) 35–40, paraffin wax 1.5–3, zinc stearate (I) 0.1–0.5, oleamidetriethanolamine oleate mixture (II) 0.5–2, ethoxylated isooctyl phenate (III), and supraparaffinated white spirits (IV) 54.1–62.7 %. It was prepared by mixing 2/3 of the oil with the wax and the compound I suspended in the rest of the oil for 2 h at 70−80 °C, added II, dispersed the mixture in IV, cooled to 25−30 °C, added III, and mixed for 15 min. The mixture on application to metal surface at room temperature, and on solvent evaporation (30−60 min), a 5−8 μm anticorrosive layer was formed which was stable for 6–12 months in an enclosed room [79]. Northan and Boies disclosed a composition for application to areas or metals where damaged paint film occurs that consists of 2 parts alkylaryl sulfonate and 1 part surfactant (barium or ethylenediamine salt of a partial phosphate ester of an ethoxylated alkylphenol) combined with a microcrystalline wax in the proportion of 90 corrosion preventive composition to 10 wax. The formulation can be dissolved in mineral spirits in the ratio of 1 part corrosive preventive composition to 3 parts mineral spirits and packaged in a pressurized spray container using Freon 12 as a propellant and can be used for spraying [80]. A paraffinic composition (viscosity 6−8 cSt at 90°) containing 25–35 % oil and 65–75 % solid paraffin was applied as a mist to surface-finished steel articles stored in packages [81]. An RP useful for hot-dip coating was prepared from petroleum type paraffin wax at 150 °C by blowing with air, and the low boiling fraction of the products separated by distillation. The products (1 kg) were partly esterified with N,N-diethylethanolamine (70.5 g) by heating at 125 °C. The esterified products (acid value 9) 11.0 parts were dissolved in 100 parts of spindle oil to give the RP mixture. Steel sheets coated with the developed RP does not show any rust formation in humidity cabinet test at 49 °C for 20 days [82]. Strippable-type coatings are well documented. An RPC (with dry thickness ∼10 μm) based on poly(vinyl iso-butyl ether) and poly(vinyl butyral), which allowed the metal to be welded without removal of the coating, was disclosed by Stegen et al. [83]. A water-displacing RP composition for crevices and microcracks of paint films on metal was prepared from 2 parts sodium/barium/ammonium alkylarene sulfonate, a 25 % solution of a surfactant (a barium or ethylenediamine salt of a partial phosphate ester) in mineral spirits, and a hydrocarbon-soluble resin in 25–50 % mineral spirit [84]. Steffers et al. disclosed an RPC useful on products containing dissimilar metals that contain filmbuilding polymers (e.g. butyl acrylate-styrene copolymer) and mixtures of zinc octanoate, oleylsarcosine, rape-oil fatty acid diethylamides, alkylnaphthalenes, and mineral oil in solvents containing 20−40 vol.% components with volatility number > 35 [85]. Temporary corrosion

2. Earlier reported works Traditionally mankind used oils and waxes to prevent rusting. Hence tracing back the first inventor of temporary RP is perhaps a difficult task. A few reports on RPs prior to 1940s are available based on oils [46–48], bitumen [49–51], petrolatum [52,53], rubber [54], and others [55–57]. Rust preventive lubricating oils were extensively applied in World War II to avoid rusting from severe humidity environments [58]. Haffner et al. in 1943 patented a chemically stable aqueous colloidal dispersion of a polycarboxyl acid containing at least 16 C atoms to protect the metal from corrosion [59,60]. In 1946, Sharp patented an RPO that made by the addition of a partial ester of sorbitan and oleic acid (5 %) and the sodium soap of oil-soluble petroleum sulfonic acids (5 %) to lubricating mineral oil. The rust preventive effect of the esters was found to be the best when only one of the eOH groups of the alcohol was esterified [61]. Pilz and Farley put forward a correlation between contact angles and rust prevention capability and demonstrated that the equilibrium surface tensions of the water and the oil and their interfacial tensions are the key aspects deciding the contact angle degree [62]. Palmer showed that a mixture of calcium sulfonate and diamylphenyl phosphate, when added to mineral oils, inhibited the rusting of metals [63]. Barnum and Larsen proposed a mechanism of protection by RPOs and the protection was attributed to the adsorption of polar molecules as oriented multimolecular layers at the steel/oil interface. The work suggested that a minimum of about six monolayers is necessary to form an efficient barrier to moisture [64]. Shearon provided an account of petroleum-based protective coatings, where he described petroleum asphalts [26]. A general classification of RPOs has been made by Clayton and Thompson [65]. Sanyal and Preston have investigated several organic and inorganic inhibitors and shown that by emulsifying a vegetable oiltype protectives with water containing inorganic inhibitors, provided proper protection [66]. Kashima and Nose through electron diffraction studies of stainless-steel surfaces coated with four kinds of anticorrosive oils showed that the molecules of oils that are recognized to give favourable property orientated on surfaces in a closed packed state, whereas at unfavourable samples only amorphous state was observed [67]. Kashima and Takuma showed that the ability of a rust-inhibitor compound to retard corrosion and remove fingerprints from steel could be deduced from the surface-potential changes of the RP-coated metal in an electrolyte solution containing chloride ions by using repeated pulse discharges [68]. Their results showed that the ordinary petroleum-oil films permit passage of oxygen or water vapor, and that these gases are then adsorbed on the metal surface. However, RPOs inhibit adsorption of oxygen or water [67]. Roden described a closed-cell equipment which gives better reproducibility and shortens the test time of RPOs. Correlation between this method and the humidity cabinet was made [69]. Morii reported light-yellow syrupy methylenebis (oleamide) as a useful RPO [70]. Chiba patented a refined wool greasebased RP. The refined wool containing 6 wt.% of Ca(OH)2 was heated for 3 h at 150 °C to obtain a product, and that was mixed with 10 wt.% wool grease and diluted with machine oil [71]. Rust inhibitors are a common additive in RP formulations. Petroleum sulfonates were one of the conventionally used rust inhibitors. In a study by Wilson, commercial sulfonates were treated with bentonitic clay at 100−450 °F to remove 5–30 % of the oil-insoluble sulfonates by adsorption. The RPO composition made with the purified sulfonates showed excellent performance [72]. A naphthenic acid (aluminium hydroxide-sodium sulfonate-naphthenic acid) based RPO was reported by Wasson [73]. Benbury and Connelly disclosed RPOs with military specifications for protection against corrosion by humidity, acid, and salt-water by adding a glycol-based synergistic agent 3

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protection on regenerative preheaters of boilers was achieved by a 1–10 % polyether and 1–10 % C6–12 fatty alcohol in mineral oil [86]. Mineral oils containing tannic acid derivatives showed excellent rust preventive characteristics. Here, 30 g tannic acid (I) was treated with 7 g benzyl bromide in DMF in the presence of NaOH to give 35 g tannic acid ester (II); 30 g of which was blended with 1000 g of a lubricating oil to obtain the RP formulation, which showed no rust on steel in salt water (JIS K 2510) compared to light rusting when I was used instead of II [87]. Steinmec et al. patented an RP agent that consists of a glycol and/or polyglycol alkenylsuccinic monoester (mol. wt. 350–650, C15-35 alkenyl group) 0.1–4, ethylenediammonium alkylaryl sulfonate (mol. wt. 400–550) 2–16, C10-40 alcohol 1–12, silicone oil ≤ 0.1, and hydrocarbon oil (viscosity 2−30 cSt) 67−98 wt.% [88]. Nowosz-Arkuszewska et al. disclosed an RPC that contain 50–98.8 parts petroleum product, 0.2–25 parts olein soap mixture with a nonionic surfactant, and ≤ 0.5 parts lead/calcium naphthenate surfactant [89]. A patented RP agent comprised of ceresin 15, diisobutyl-p-cresol 0.1, 20 % oil concentration of polyisobutylene rubber 1.5, and reaction products of diethylenetriamine, alkylarylsulfonic acid (mol. wt. 560), and propionic acid 6, and balance mineral oil (kinematic viscosity 19.6 cSt at 100 °C). The RPC coated steel showed no corrosion when exposed to 3 % NaCl solution for 300 h [90]. An RP agent containing petroleum hydrocarbons 50–59, corrosion inhibitor 1–10, and glycerides of higher fatty acids (surfactants) 4–40 % was disclosed. The product comprised of melt prepared from ceresin 35, paraffin 35, zinc stearate 5, and fatty acid glycerides 25 parts [91]. Schreier et al. reported a lubricating RPO that consists of an oil component 93–98.6, reaction product of alkylarylsulfonic acids (mol. wt. 350–500) with Ba(OH)2 1–5, and chosen components from oleic acid diethanolamide, olein, oleoylsarcosine, and N(CH2CH2OH)3 salts 0.4−2 wt.% [92]. A patented composition to remove water from the surface of ferrous materials after pickling and rinsing, to prevent the formation of yellow stains, and to give corrosion protection during storage comprised of neutral mineral oil 150–170, n-butyl alcohol 150–200, triethanolamine 10−15 mL, and calcium stearate 125−150 mg [93]. Iwanow et al. reported a nontacky water-removable and high-temperature stable (≤ 120 °C) composition with excellent adhesion comprised of aluminium stearate 8–20, phenolic resin 8–20, lubricating oil 8–20, oleamide 0.1–1, petroleum spirit 40–80, and alkyd resins 0–10 parts [94]. Kekenak et al. patented a composition that contained refined mineral oil, polyisobutylene thickener, calcium and sodium mahogany soaps, calcium alkylsalicylate, silicone oil, and oxidized rape seed oil. The composition optionally contained petrolatum, diethanolamine, and/or succinimide [95]. A disclosed agent for rapid drying and temporary corrosion protection of moist surfaces of metals following machining in aqueous media or etching contained barium tetracosylbenzene sulfonate (I) 10–25, benzyl alcohol or furfuryl alcohol 2–8, anhydrous lanolin 2–8, kerosine 5–78, and 1:1 turbine oiltransformer oil 8 % [96]. An RPO having steroids and their derivatives partially esterified with fatty and rosin acids along with wax was patented by Marczewski et al. [97]. As discussed above, several reviews/ articles on RPs are available published during these periods [5,22–33].

A lubricating oil without speciality additives would permit faster rusting initiation most likely in a few hours of application as moisture displaces oil more efficiently. It is expected that certain dissolved oxygen present in the oil permit rusting to occur if the film is defective. With a thicker coating, it is likely that with increase in time, loss of film continuity may happen due to the appearance of small fissures or imperfections in the coating. Hence speciality additives such as the rust inhibitors are essential [26,103]. The most commonly employed rust inhibitors include polar compounds such as sulfonate metal salts (calcium and barium petroleum sulphonate), sulfonate amine salts, magnesium/barium salts of higher fatty acids, carboxylic acids, esters, amines, soaps of aliphatic/naphthenic/sulphonated acids and lanolin. The degree of the rust inhibition is determined by the average lifetime of the formed film, the film permeability to water/aqueous ions, and the capability of the reaction products to diffuse away through the film. Addition of wax in addition to the inhibitors can enhance the rust preventive effect by thickening the coating [26,103]. The additives in RPs need to be selected carefully to avoid destructive interference; for example, certain rust inhibitors can promote the film oxidation, or can cause oil-water emulsion formation. The additives need to help increase the wetting speed, inhibit film oxidation, upsurge the film strength, enhance adhesion characteristics and augment lubricating qualities [6,26,32]. Other organic and inorganic inhibitors can also be used in formulations depending upon the nature and type of application. Pompowski et al. studied the role of surface-active radicals in corrosion inhibitor molecules on the protective effect of RPOs and showed that oily layers that contained inhibitors had a higher active resistance and enhanced firmness and, diminished the corrosive current significantly. The improved inhibitive properties were associated with the occurrence of unsaturated bonds in the molecule [104]. Influence of change in the molecular structure of hydroxyarylstearic acids on its efficiency as a combined oxidation-rust inhibitor in bis(2-ethylhexyl) sebacate was investigated by Snead. The results showed that alkyl groups present in the ortho and para position to the hydroxyl of hydroxyarylstearic acids improved oxidation protection but reduced rust protection. A 9,12-bis(4-hydroxyphenyl)stearic acid was found to be the best antioxidant, with only a slight negotiation in rust protection [105]. The rust inhibitors in general dissolve in oils forming various mixed micelles. Nose and Watanabe studied the degree of interaction of sodium dinonylnaphthalene sulfonate with other rust inhibitors like potassium abietate and lead naphthenate. The activity of the mixtures of rust inhibitors in a commercial RPOs was found decreased by the formation of mixed micelles [106].The results of accelerated corrosion tests by Helwig showed that the rust resistance of oiled sheet is primarily a function of the chemical composition of the rust inhibitors and the amount of rust inhibitor deposited on the metal surface [107]. More details on organic rust inhibitors, their surface adsorption, and the role of critical micelle concentration can be found somewhere else [108–113]. The following section describes works reported on RPs from 1985 to the present.

2.1. Rust inhibitors in RPs

3. Recent R & D

The theory of RPOs based on the orientation and the monolayer deposition of polar compounds on the metal surface was well described from the 1940s itself [23,25,32,62,98–101]. In RPOs, the actual protection is provided by the polar additives, via the formation of nearly close-packed and vertically oriented molecular monolayers by adsorption at the metal-oil interface. They form semi-rigid lattice along with the oil at the metal–oil interface. Bulkey and Snyder postulated the existence of such adsorbed layer from experiments using oleic acid [102]. These rust inhibitors have in general have a large hydrocarbon group attached to one or more smaller polar groups. Good reviews on polar type inhibitors are available [29,32].

More than 90 % of the recent works reported in this area are patents (see § 3.1). There are only a few journal publications. For example, Khaire et al. compared four types of RPOs which consists of different carriers like mineral oil (A), solvent (B), mix of solvent and oil (C) and oil with VCI addition (D). The results of the study showed that RPOs with VCI additives (mineral oil base carrier with amines and VCI additives) were the best in providing rust protection. This oil (D) was easy to apply as well with lesser viscosity, and good lubricity. Sample A also performed reasonably well. It had higher viscosity due to thixotropic property; the lubricity was not good. Samples B and C not well-performed as expected. Sample C was comparatively better to Sample B. It 4

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was also shown that mineral oil with amorphous wax gave more extended protection than the solvent and solvent mixture type carriers [17]. For RPOs, typically the base oil can be from mineral oil, synthetic oil or vegetable oil or their mixtures. As discussed above, the rust inhibitor can be selected from sulfonic acid salts, sarcosine-type compounds, esters, amines, carboxylic acids, fatty acid amine salts, carboxylic acid salts, alkyl- or alkenyl-succinic acid derivatives, boron compounds etc. [114]. An anti-rust oil composition patented for cutting tools by Mao includes anti-rust agent, base oil, corrosion inhibitor, antioxidant, and corrosion inhibitor auxiliary agent [115]. Li et al. patented a preparation method of multifunctional antirust oil where the steps include: preparing raw materials (base oil, film-forming agent, oil solvent, antirust agent A and antirust agent B) and then dispersing antirust agent A and film-forming agent in mineral oil, obtaining the mixture and dispersing the oil solvent, antirust agent B and base oil in the mixture. The synthesis strategy yielded RPOs with proper viscosity, good anti-rust performance and excellent water displacement performance [116]. A patented method of RPOs with low water content comprised the following steps: (1) heating and stirring base oil 40–80 %, and raising temperature to 50−100 °C, (2) adding given amount of rust inhibitor and antioxidant, heating to 95−130 °C, and stirring for 0.5−6 h at constant temperature to obtain a mixture, and (3) adding the rest base oil into the mixture, and stirring to obtain the final product. The composition contained rust inhibitor 5–30 %, antioxidant 0.1–5.0 %, and base oil 65–93 % [117]. Ghanbarzadeh et al. studied the influence of oil type and reaction temperature on the sulfonation of base oils. They have used three types of industrial mineral base oils (SAE10, SAE30, SAE40) that were dipcoated on steel panels. The incorporated sulfonate-based inhibitors were expected to form densely packed layers with hydrocarbon chain orbited outward from the metal surface (Fig. 2a). The initial humidity cabinet studies showed that the protection performances of the three base oils were inferior to two commercial RPCs (Ref 1 & 2 in Fig. 2b). As a next step, the inhibitor agents were prepared by sulfonation of base oils at various temperatures with oleume as reactive agent. The sulfonation efficiency of the different base oils at different temperatures are

shown in Fig. 2c. The sulfonate groups were neutralized with calcium hydroxide to attain hydrophilic calcium sulfonate groups and are used as inhibitor agents in base oils. The corrosion resistance efficiency of dip-coated steel panels was studied in 100 % relative humidity cabinet. The results showed that for the best RPO composition, the first rust stain was observed only after 720 h in humidity test (Fig. 2d). The study further confirmed that sulfonation process can be used for augmentation of protective effect of petroleum oils [16]. Melnikov et al. tested various perfluoropolyalkylether derivatives as corrosion inhibitors in RPs. The results showed that structural differences between the different derivatives of the compound led to noteworthy variation in their inhibiting effect. The optimum of protective properties was observed when perfluoropolyalkylether chain length was 5–7 [118]. The following section provides a comprehensive description of patented works during the period 1985 to the present. The subsections are classified based on the major component in the formulation even though such a classification is not impeccable. More details on works reported other than patents are included in § 4. 3.1. Recent patented works 3.1.1. Fatty acids-based Several recent patents are available with various fatty acid-based RPs (fatty acids other than that present in base oils) [119–131]. Nrihito patented a composition that comprises of base oils, lanolin fatty acid derivative, and vegetable oils and fats. The composition does not form precipitates when contaminated with water-soluble metalworking oils [119]. Fatty acids of tall oil at 97.0–99.0 %, and pine-oil fat-soluble concentration at 1.0–3.0 wt.% was patented by Trusov et al. [120]. A formulation containing amine salt of fatty acid 40–80 %, non-ionic surfactant 4–10 % and hydrocarbon base oil 56-10 % was disclosed by Kaul et al. [122]. Another patented composition comprised of lowgrade lignosulfonates 6–10, a mineral oil 2–5, a waste component (collected from the production of C10-C17-fatty acids) 60–65, and an additive mixture of primary C8-C11-amines 24–28 %. The composition has improved anticorrosive properties due to high adhesion and reduced porosity of the coatings [123]. Matsuzaki and Sugawara

Fig. 2. (a) Orientation of surfactant molecules, (b) the time for rust initiation with different base oil coatings, (c) Effect of oil type and reaction temperature onto sulfonation reaction, (d) effect of oil type and volume % of surfactant on rusting [16]. 5

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disclosed an RPO composition comprised of (A) -OH-containing compounds of (a) C10-22 aliphatic monohydric alcohols and/or (b) partial esters of C10-22 fatty acids with glycerin/trimethylolethane/sorbitan 0.5–10.0, (B) sulfonic acid alkali metal salts 1.0–15.0, and (C) oxidized wax metal salts, lanolin fatty acid metal salts, and/or lanolin fatty acid esters 1.0–15.0 % [124]. An anti-rust oil comprises of base oil, poly-αolefin, lanolin, dodecyl succinate, 2,6-ditert-butyl-p-cresol, 3-fluoro-4methoxybenzyl chloride, polyethylene glycol 400, fatty acid pentaerythritol ester, alkenyl azelaic acid, chitosan, barium petroleum sulfonate, oleic acid triethanolamine, dioctyl phthalate and dialkyl dithiophosphate zinc was patented by You et al. [125]. Sakakibara and Imaeda disclosed a technology for applying a novel RPO on iron products coated with iron oxide. The composition comprised of a rust preventive agent that contained a fatty acid metal salt, metal sulfonate, and ester compound, a first base oil with a kinematic viscosity of 2−50 cSt at 40 °C, a second base oil with a kinematic viscosity of 300−600 cSt, and an organic solvent. The proportion of the rust preventive agent with respect to the whole RPO composition was 15–30 mass%; the proportion of the first base oil was 2–10 mass%, and the proportion of the second base oil was 28–50 mass% [126]. A patented slushing composition comprised an alkaline petroleum calcium sulfonate 10–50, 4-methyl-2,6-di-tert-butylphenol 0.5–10, and a reaction product of C17-20 synthetic fatty acids with aliphatic amines in a medium of C12-28 α-olefins 47−87 wt.%. The composition can also contain a fraction of petroleum oil [127]. Several patents made use of oleic acid and its derivatives. Chen reported a smooth RPO that includes: 300−500 wt. parts of trihydroxymethyl propane oleate, 200–400 parts of multifunctional antirust agent, 10–30 parts of fatty alcohol polyoxyethylene ether-9, 450–650 parts of paraffin base oil, 40–60 parts of sulfurized olefin cottonseed oil, 10–30 parts of polyisobutene, and 5–10 parts of an antioxidant [128]. A patented RPO by Zhang and Zhu consists of magnesium stearate 13−14 wt. parts, sodium naphthenate 5–8, ethyl silicate 3–5, potassium octadecyl polypropylene ether sulfonate 1–2, oleic acid 83–89, and lauric acid 11–14 [129]. Zhu disclosed an RPO that comprises of hydroisomerization base oil 78−85 wt.%, vegetable oil (olive oil/ peanut oil) 7–12, iso-propyl oleate 5–10 and antioxidant T501 0.1–0.2. The RPO has environmental protection, no odor, high safety, long rust prevention period, no wastewater pollution, small viscosity, convenient coating, and large application prospect [130]. Fu discloses super-long weather-resistant and salt fog resistant RPO, which is prepared from rare-earth antirust hydraulic oil 14−20 wt. parts, hydroxamic acid 0.7–1, 30# mech oil 60–70, gear oil 10–20, diatomite 0.6–2, anti-aging agent 0.3–1, sodium salicylate 1–3, oleate potassium soap 4–10, polyoxyethylene oleate 0.5–3, epoxy soybean oil 3–6, neutral barium dinonyl naphthalene sulfonate 3–5, chitosan 0.2–1, and diallylamine 0.2–2. The invention adds rare earth antirust hydraulic oil, uses N-vinyl pyrrolidone dispersant to improve mobility and enhance reactivity, and can form an insoluble complex through rare-earth ions with OH− generated on the metal surface [131].

sheets that was composed of base oil 70−80 wt.%, a sulfonate compound 7–13, an ester derivative 5–10, a carboxylate substance 2–5, oxidized wax 0.4–2, a carbonate substance 0.5–3, a triazole 0.1–1, a phenol derivative 0.1–1 and an acrylate 0.1–1 [133]. Nam disclosed an RPO for steel sheet that comprises of base oil 70–85 %, sulfonate-based material 3–20, ester-based material 1.0–10.0, overbase calcium sulfonate 1–10 %, and aliphatic alcohol 1.0–10.0% [134]. Okada et al. patented an RPO for steel, having base oils, oxidized wax metal salts 15–25 %, sulfonates of ≤ 20 mg KOH/g base no. 1–5 %, sulfonates of ≥ 30 mg KOH/g base no. 2–8 %, and metal soaps of lanolin 1–3 %, and have 15−25 cSt kinematic viscosity at 40° [135]. Takeshima and Ohnishi presented an RPO that contain (A) ≥1 alkyl amines (with C6-24 alkyl groups, preferably C8-18 alkyl groups) 0.1–30, (B) ≥1 compounds selected from basic or neutral, alkali or alkaline earth metal salts of aromatic sulfonic acids, polyol partial esters, and metal salts of partial esters of oxidized waxes 1–30, and (C) a base oil (having viscosity of 0.5−20 cSt at 40 °C) 40–98.9 wt.% [136]. Niu patented a synergistic multicomponent RPO for application after water washing and cleaning that composed of the following components: barium petroleum sulfonate 2–3, lanolin magnesium soap 1.5–2, 2-ethylhexanol 2–3, sorbitan monooleate 0.1–0.25, polycetyl methacrylate 0.2–0.3, sulfonated lanolin sodium 1.5–2.5, iso-propyl palmitate 0.1–0.2, polyoxyethylene sorbitan monooleate 1.5–3, sorbitan fatty acid ester 2–4, 2,6-di-tertbutyl-4-cresol 0.2–0.4, sulfuric acid butanol octanol zinc salt 2–3, heptadecenyl imidazoline oleate 1–1.5, dodecenyl succinic acid semialuminum soap 1–1.6, pentachlorobiphenyl 0.3–0.5, N-oleyl sarcosine octadecyl amine salt 0.5–1.5 wt.%, and transformer oil as balance. The RPO showed superior antirust and lubricating effect and was particularly suitable for anti-rust protection of water cleaned devices [137]. Zhang et al. disclosed an RPO that comprises the following components : 75–90 parts by wt. of base oil (mineral oil or hydrogenated oil), 5–15 of oil-soluble rust inhibitor (selected from at least one of barium petroleum sulfonate or synthetic barium sulfonate), 0.5–1 synthetic ester (monoesters, diesters or mixed ester), 0.5–1 extreme-pressure antiwear agents, and 0.1–1 other additives. The composition was suitable for storage and rust protection of bearing products and vibration/noise reduction of bearing test [138]. A patented RP composition contained mineral oils and/or synthetic oils as base oils and (1) neutral alkali metal and/or alkaline earth metal sulfonates 0.1–20, (2) basic alkali metal and/or alkaline earth metal sulfonates having total base no. 20500 mgKOH/g 1.0–30, and (3) oxidized waxes and/or their derivatives, polyol partial esters, and/or lanolin fatty acid derivatives 0.1–20 % and have kinematic viscosity of 1−150 cSt at 40 °C, total base no. 230 mgKOH/g, and total base no./total acid no. ratio 4-25. The composition helped prevent rust in acidic atmosphere [139]. An economical and stable piston rod RPO disclosed comprised of lead naphthenate 5−10 g/L, zinc naphthenate 8–12, sodium petroleum sulfonate 18–23, barium petroleum sulfonate 20–25, calcium petroleum sulfonate 16–20, potassium oleate 10–15, and gasoline [140]. Zhang patented an environment-friendly bearing anti-rust oil, which is composed of 70–88 % base oil, 5–10 film-forming agent, 2–5 rust inhibitor, 2–5 calcium dinonylnaphthalene sulfonate, 1–3 alkali base calcium sulfonate, 2–5 dodecylsuccinic anhydride derivative, 0–1 antioxidant, 0–1 organic alcohol ether [141]. A water displacing type RPO consisted of petroleum barium sulfonate 4, benzotriazole 0.1, barium soap 2, petroleum sodium sulfonate 1, zinc naphthenate 0.2, lanolin 2, machinery oil 15, and kerosene 75 wt. parts, where kerosene was filtrated through 5 μm filter bag before use. The invention solves the problem of dust forming in the metal device storage and processing processes [142]. A patented dewatering RPO was prepared from base oil 75−85 wt.%, synthetic sulfonate 0.5–10, naphthalene sulfonate 0.5–10, lanolin Mg soap 0.5–10, and a dewatering agent 10–15. The production method comprises under normal pressure, adding base oil in a container, heating to 120 °C, stirring, vacuum-dewatering for 15 min, adding lanolin magnesium soap, naphthalene sulfonate and synthetic sulfonate, complete dissolving, stopping vacuum and heating, lowering

3.1.2. Sulfonate as RP agent High molecular weight sulfonates remain to be a main component in RPO formulations. They provide good water separation characteristics. The most important role of sulfonates is possibly the surface passivation of the metal surface via chemisorption of the polar head groups. One of the widely employed sulfonates is barium petroleum sulfonate/synthetic barium sulfonate. Ding discloses a rust prevention treatment method that includes the following steps: (1) water washing at 80−90 °C for 20−30 min; (2) drying in an oven at 100−105 °C for 5−10 min; and (3) performing rust-proofing treatment by soaking in RPO for 2−4 min. The RPO was composed of barium petroleum sulfonate 2–4, sodium dodecyl benzenesulfonate 1–3, lanolin 2–6, sorbitan monooleate 0.1–0.3, rust inhibitor 0.5–1.5 wt.% and mech oil the balance [132]. Kim patented an RPO with long term rust preventive property for galvannealed steel 6

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the temperature < 50 °C, adding dewatering agent, stirring to complete dissolving, and cooling to normal temperature [143]. A patented slushing composition for temporary protection of metal surfaces contained an esterification product of triethylene glycol and an oxidized hydrocarbon fraction (pour point of 308 K, acid no. 3-4 mg KOH/g, pH of aqueous extract 5–7) 0.3–20, a corrosion inhibitor having calcium C12-13 alkylbenzene sulfonate (Ca content ≥ 5 wt.%, S content ≥ 8 wt.%, acid no. ≤ 25 mg KOH/g) 0.1–5, in addition to mineral oil (kinematic viscosity at 323 K (10–14) ×10−6 m2/s, pour point ≤ 228 K, acid no. ≤ 0.07 mg KOH/g) 0.1–65, lanolin (acid no. ≤ 8 mg KOH/g, pour point 305−321 K, saponification no. 70–140) 0.1–5, and a petroleum solvent (S content ≤ 0.1 %, acid no. ≤ 0.6 mg KOH/100 cm3) 5−60 wt. parts [144].

epoxidized soybean oil 21–26, petroleum acid 6–9, bactericide 4–7, otoluidine 6–8, and sodium hexametaphosphate 9–10. The preparation method comprises the following steps: taking sliced radishes for fermentation, sterilizing and filtering; taking tea leaves for grinding, fermenting, adding the fermentation liquid into the filtrate of last step, fermenting, filtering, heating, filtering, mixing the filtrate for two times, adding bactericide; taking disodium hydrogen phosphate, zinc oxide, zinc nitrate, magnesium chloride, glycerol, phosphoric acid, otoluidine, and the mixture of last step, sterilizing, adding defoamer, silicon dioxide, glycerol, manganous dihydrogen phosphate, polyethyleneglycol, petroleum acid, sodium hexametaphosphate, and epoxidized soybean oil, and reacting at 21−25 °C for 18−20 min to get the final product. The agent can effectively remove rust and other dirt attached to the steel surface and can leave an RP film on the surface [150]. Liu et al. disclosed an environment-friendly anticorrosive treatment agent for screw-threaded steel surface, that is prepared from the following raw materials: zirconium salt 5−15 wt.%, sodium hypophosphite 2–8, alkali 0.5–5, corrosion inhibitor 0.1–0.5, stabilizer 0.1–0.5, nickel fluoride 0.05–0.08, phenyl ethylene glycol ether 0.5–0.8, tertbutylhydroquinone 0.05–0.1, monoglyceride stearate 0.05–0.08, 4sulfophthalic acid 0.05–0.08, 2-sulfophenylpropiolic acid 0.02–0.05, and water as balance. The environment-friendly anticorrosive treatment agent has obvious antirust effect, long antirusting time, low cost, no pollution and high safety [151]. A patented water-based agent comprised of 3−5 wt. part of monoethanolamine benzoate, 2–4 triethanolamine, 0.5–2 thiourea, 1–3 sodium molybdate and 25–40 water; it also comprised 1–3 part of carbohydrazide, 0.5–1 octylphenol polyoxyethylene ether, and 1–2 dimethylethanolamine. The preparation method includes orderly adding monoethanolamine benzoate, triethanolamine, thiourea and sodium molybdate into the water, thoroughly stirring and dissolving to obtain the special aqueous rust inhibitor for cold-rolled sheet package; orderly adding carbohydrazide, octylphenol polyoxyethylene ether and dimethylethanolamine, thoroughly mixing, and ultrasonically dispersing for 10 min. The composition can be applied by electrostatic spraying, dipping, or brushing method, forming a layer of protective antirust film on the surface of the cold-rolled sheet, and subsequently enclosing it in a plastic bag, and preserving under sealed conditions [152]. Metal sheets were treated with an aqueous solution containing ≥ 1 polyethoxylated fatty alcohol or fatty acid and an oil-in-water emulsion containing 3−13 vol.% oil phase consisting of mineral oil 75–90, a surfactant 5–10, and optionally a corrosion inhibitor 5−15 vol.% to prepare surfaces for stamping and provide temporary corrosion protection [153]. A patented temporary waterborne coatings prepared for improved weather and sea water resistances was made of 40–44 % aqueous vinyl chloride-vinylidene chloride copolymer dispersion 31–35, poly(vinyl alcohol) 17–20, glycerol 6–9, spindle oil 12–15, Aerosil 4–5, sodium alginate as corrosion inhibitor 3–5, sodium tripolyphosphate 0.3–0.8, sodium pentachlorphenoxide 0.2–0.6, diethylene glycol 1–2, and zinc powder 4–7 %, with the remainder being water [154]. Speckmann et al. disclosed a stable low-viscous oil-in-water emulsion for temporary corrosion protection of ferrous metal surfaces that comprised of (1) mineral oil (Pionier 4556 40) 10–60, (2) an emulsifier (Eumulgin B1) consisting of ≥ 1 ethoxylated C10-22 fatty alcohol 1–10, (3) a corrosion inhibitor consisting of ≥1 RCO2H (R = C6-22 alkyl or R1C6H4COCH:CH-; R1 = C8-18 alkyl) [1:1 stearic acid-palmitic acid mixture was used] 1–10, (4) a co-emulsifier consisting of ≥ 1 C12–22 fatty alcohol 0−10 wt.%, and (5) water as balance. It was prepared by emulsifying all liquid components and heating the emulsion or emulsifying the mixture at a temperature within or above the phase inversion temperature range, cooling the emulsion below the range, and diluting with water. When the coated specimens were exposed to flowing air having a relative humidity of 100 % at 50 °C, 100 % corrosion was observed only after 40 days whereas specimens coated with a conventional emulsion exhibited 100 % corrosion after 13 days [155].

3.1.3. Sarcosine as RP agent Recently, sarcosine and its derivatives are investigated as a novel rust preventive component [114,145–149]. Sarcosine (N-methylglycine) is an aminoacid derivative and is an intermediate and byproduct in glycine synthesis and degradation. It is water-soluble and biodegradable. Motoyama et al. patented a composition that contain base oils (mineral oils and/or synthetic oils) and sarcosines represented by [R1CONR 2(CH2 )n CO2 ]X, [R1 CONR2 (CH2)nCO 2] iY, or [R1 CONR2 (CH2) nCO 2] m Z(OH) m'; where R1 = C 6-30 alkyl/C6-30 alkenyl; R2 = C1-4 alkyl; X = H/C 1–30 alkyl/C1–30 alkenyl; Y = alkali/alkaline earth metal; and Z = polyhydric alcohol residue; when Y = alkali metal, i = 1; when Y = alkaline earth metal, i = 2; m, m' ≥ 0; m + m' = valence of Z; n = 1–4. The composition retained high rust prevention property for a long time and showed good degreasing property, and atomizing property [145]. An RPO composition comprising a first mineral oil (kinematic viscosity ≥ 6 cSt at 40 °C), a second mineral oil (kinematic viscosity ≤ 250 cSt at 40 °C), a fatty acid amine salt, an ester, and at least one rust preventive agent selected from the group comprising sarcosine compounds, sulfonates, esters, amines, and paraffin wax was patented by the same group [147]. A disclosed RPO composition consists of the following raw materials in parts by wt.: 550sn base oil 85, tetrahydrofurfuryl alcohol 3, calcium petroleum sulfonate 7, titanate coupling agent TMC-105 2, lithium carbonate 3, caprylic/capric triglyceride 5, sodium benzoate 3, fatty alcohol polyoxyethylene ether 3, dodecenyl succinic acid half ester 5, aluminum stearate 3, p-nitrophenol 2, N-oleyl sarcosine octadecylamine 6, and antirust hydraulic oil 25 [148]. Cai et al. disclosed a longlasting RPO for iron casting, which comprises base oil 90−110 wt. parts, calcium petroleum sulfonate 3–5, N-oleyl sarcosine-2-aminoethylhexadecenyl imidazoline 1.5–2.5, ricinoleic acid 0.1–0.4, benzotriazole 0.5–0.8, triglycerol monostearate 1–3, alkylphenol ethoxylates 0.2–0.5, octadecyl methacrylate 1–2, silane coupling agent KH540 0.2–0.4, t-znow 0.05–0.15, nano rectorite 0.05–0.1, and phenolic antioxidants 0.6–1.2. The RPO has the advantages of good corrosion resistance, good heat and moisture resistance, good protection effect on the surface of iron casting, and long rust prevention time [149]. Shibata et al. disclosed an RPO composition containing a nonionic surfactant (0.1–10 mass%) having a hydrophile-lipophile balance of 10–12 and at least one rust inhibitor selected from among sarcosinetype compounds [114]. 3.1.4. Water-based Several patents on water-based RPs are available [150–156]. As discussed above, they are attractive in terms of environmental regulations. Wang et al. disclosed a water-based treatment agent for steel, which comprised of disodium hydrogen phosphate 19–24 parts, glycerol 24–28, phosphoric acid 55–63, zinc oxide 15–25, zinc nitrate 40–46, magnesium chloride 7–15, defoamer 3–7, tea leaves 33–38, sliced radishes 23–26, silicon dioxide 13–16, glycerin 22–25, manganous dihydrogen phosphate 11–14, polyethyleneglycol 16–19, 7

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A safe and practical mixture reported for temporary corrosion protection of iron parts comprises water 60–80, mineral oil 5–12, alkali silicate 2–8, chloroparaffin (Cl content ∼30 %) 2–6, paraffin 1–5, mepasine sulfamide 1–3, an ethylene glycol derivative 1–2, C10–14 unsaturated fatty acids 0.5–3, and poly(vinylacetate) 0.5–2%. The efficiency of the mixture with malleable cast iron fittings was two times higher than that of a pure paraffin oil [156].

antioxidant (e.g. 2,6-di-tert-butyl-4-methylphenol), thixotropic agent (e.g. hydrogenated castor oil) and antirust agent T705, and stirring. The obtained film has high hardness and anti-rust property [164]. Kronstein showed that by dispersing complexes of Zn or ZnO with lecithin, alkyd resins, or polyurethanes in nondrying oil yield temporary RPs for steel [166].

3.1.5. Resin-based A few patents addressed resin-based RPCs [157–166]. Nakane disclosed a resin composition containing 15 % cycloolefin resin (Topas 9506F04) and 8.5 % xylene that was applied on a cold-rolled steel sheet and dried to form a coating, which was peeled after 240 h of test (49 °C, relative humidity 95 %) where no discoloration or corrosion noted to the base metal [157]. Hu et al. disclosed a composition made of base oil 80–100 parts, petroleum resin 20–30, tert-butylphenol formaldehyde resin 30–40, diluent 20–30, anhydrous ethanol 10–20, a rust-proof agent 10–20, antioxidant 3–5, dry oil 8–10, organosilic resin 8–10, modified additives 20–30, and di-n-octyl phthalate 3–5 [158]. Yuan reported an RPC that comprised of alkyd resin 6−10 wt. parts, oxidized petroleum ester barium soap 4–8, castor oil 2–6, calcium petroleum sulfonate 8–15, Span 80 0.5–1, polyglyceryl fatty acid ester 0.2–0.8, dibutyl phthalate 1–5, and sodium petroleum sulfonate 1–5. The preparation method includes heating the alkyd resin, oxidized petroleum ester barium soap, castor oil and di-butyl phthalate to 60−80 °C, stirring well, adding Span 80 and polyglyceryl fatty acid ester, stirring until the mixed solution is uniformly dispersed, stirring for 10−20 min, adding calcium petroleum sulfonate and sodium petroleum sulfonate, keeping at 70−80 °C, and stirring for 10−20 min. The resulted RPC showed good stability and long antirust cycle [160]. Lutze-Birk et al. patented a composition comprised of poly(butyl methacrylate) 10–80, synthetic phthalic resins 1–50, mineral oil (hydrorafinate-5) 0.1–20, fatty acid esters with polyhydric alcohol (e.g., sorbitan oleate) plasticizer 1–5, and optionally, a silicone emulsion antifoaming agent 0.001–1, polyethylene wax (a wax obtained by partial depolymerization of polyisobutylene) 0.1–20, and p-cumylphenol 0.1–5 % [161]. An RP preparation method includes mixing naphthene base white oil 75−90 wt. parts, rice bran oil 4–7, coumarone resin 5–10, petroleum resin 3–6, tert-butyl phenol-formaldehyde resin 2–5 and ethanol 10–20, and adding to a reactor, heating to 80−100 °C, and stirring for 2−3 h; cooling to 55−60 °C, subsequently adding dodecenylsuccinic acid 0.5–1, calcium petroleum sulfonate 0.3–0.5, sulfonated lanolin calcium soap 0.2–0.4, gum rosin 0.2–0.5, oxidized petrolatum 0.1–0.3, oleic diethanolamide 1–3, octadecanol 0.5–1, lanonol 0.3–0.6 and benzotriazole 0.4–0.8, and stirring for 1–1.5 h at a rotating speed of 150−250 r/min; ultrasonically treating for 10−30 min; and cooling to room temperature, and filtering. The hard film RPC had good stability, no layering and settling phenomena, and excellent rust preventive property [162]. A disclosed strippable and easy recovery of hard film RP comprised the main film-forming agent, complement film-forming agent, antirust agent, plasticizer, antioxidant, other additives and solvents. The preparation procedure was as follows: adding the main filmforming agent and complement film agent into the solvent, stewing or stirring and soaking for 16 h, after completely dissolving of the film agent, heating to 45−55 °C, successively adding antioxidant, antirust agent, plasticizer and other additives, mixing the system to a uniform and transparent state and obtaining the product [163]. Chen et al. reported a preparation method of hard film RP with high hardness that includes (1) mixing light diesel oil, beeswax, turpentine, sunflower seed oil, polyoxyalkylene diol, castor oil-propylene oxide polymer, triethanolamine, ozocerite, ethyl acetate, and nonionic surfactant (fatty alcohol polyoxyethylene ether AEO-9), stirring, and heating while holding to obtain mixture I; (2) mixing barium petroleum sulfonate, ethylene glycol, diethylene glycol, kerosene and styrene-acrylic emulsion, heating while holding, and stirring to obtain mixture II; and (3) mixing I and II and adding defoamer (e.g. polyether defoamer),

3.1.6. Wax-based / others As discussed above, waxes form a major component in RPs. Motoyama disclosed a polyfunctional hydrocarbon oil composition having n-paraffin content of 10–90 mass%, aromatic content of 0−3 vol.% and naphthene content of 0−20 vol.%. The formulation improved safety (volatility, inflammability, odor and skin irritation), and functionality of the hydrocarbon oil [167]. Calcium and barium soaps of acids obtained by hydrocarbon wax oxidation can act as effective corrosion inhibitors in RPO formulations [168]. Repka et al. reported a corrosion inhibitor suitable for temporary protection of iron alloys during storage and transportation that consists of C12-22-alkylamine 10–35, a product of aminolysis of natural fats (e.g., sunflower oil, rape oil etc.) 5–40, C11-13-alkylbenzenesulfonic acid 15–45, and oxidized paraffin wax 8−55 wt. parts. The best combination was comprised of a mixture of octadecylamine 90, dodecylamine 2, tetradecylamine 2, hexadecylamine 4, 2 % eicosylamine 10, a product of aminolysis of rape oil with monoethanolamine (I) 40, alkylbenzenesulfonic acid (alkyl ≃ 11.7 C) 30, and oxidized paraffin wax (acid no. 90 mg KOH/g) 15 wt. parts in the form of a 3 % solution in ligroin. The solution was deposited on steel sheets and exposed to air having a relative humidity of 100 % at 40 °C where no corrosion was observed for 5 days whereas the unprotected sheet corroded after 24 h [169]. A barium-free environmentally friendly thixotropic RPO has comprised of base oil 73–96 %, barium-free rust preventive agent (e.g. calcium petroleum sulfonate, lanolin, oxidized wax) 2–20, thixotropic agent (e.g. hydrogenated castor oil, organic bentonite, waxes) 0.5–5.0, aid (e.g. Span 80, imidazoline, triethanolamine oleate) 0.5–5.0 and antioxidant (e.g. 2,6-di-tert-butyl-4-cresol, alkyl diphenylamine, zinc dialkyldithiophosphate) 0.1–5.0. The production method comprises adding base oil in a reactor, heating under stirring, adding barium-free RP, heating to dehydrate at 100−120 °C under stirring, cooling to 80 °C, adding thixotropic agent, aid and antioxidant, stirring, and performing 1 μ filtration. The RPO has both rust prevention ability and lubricating property. It also has excellent thixotropy which allows the formation of stable oil film on metal surface, to reduce oil flow, thereby, save oil and improve the operating environment [170]. An oil-based metal cutting fluid with high corrosion prevention performance with an incorporated modified RPO was disclosed by Motoyama et al. [171]. Many reported formulations consist of esters and carboxylic acids as major components [172–175]. Shibata and Motoyama patented RPOs based on perfluoroalkyl compounds [176]. A kerosene-based RPO comprised of kerosene 62−65 wt.%, zinc naphthenate 5–6, vinyltriethoxysilane 2–3, dodecenyl succinic acid 2–3, diethylene glycol monoethyl ether 3–5, petrolatum 0.2, and 2,6-di-tert-butyl p-cresol 0.2 [177]. Ju patented an RPO composed of base oil 18−30 wt.%, rust preventive agent 7–10, lanolin 0.5–3, corrosion inhibitor 1–5, oxidation inhibitor 0.2–0.5, organic alcohol 1–5, and solvent naphtha to balance. A coated gray cast iron remained protected for 168 h in salt spray and 30 days in direct atmospheric exposure. In the subsequent cleaning process, the film was easier to clean and remove, without causing excessive pollution and waste cleaning [178]. As discussed before, several patents used naphthanates as a major component [85,89,159,167,177,179,180]. Another interesting candidate is glycerol that found to be a successful component in multi-component rust preventive formulations [150,181,182]. RPOs were also used in special applications such as in concretes [183,184]. 8

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3.1.7. VCI-based Several patents were reported on VCIs as alternative to RPOs [185–190]. For example, Cho disclosed a volatile corrosion prevention industrial packaging wrap that can effectively prevent corrosion of the packaging products without using RPOs [185]. On the other hand, many formulations make use of synergism between RPO and VCI. Takahashi patented a volatile anticorrosive solution composed of a volatile anticorrosion agent mixed with an RPO and/or an alcohol solution [186]. A patented vaporizable RPO contained (A) 2−40 wt.% of aminebased composition (100 parts of carboxylic acid amine salts and 10–50 parts of ethanolamines), (B) 1–30 morpholines, and (C) 30−97 glycol ethers. The oil exhibited excellent degreasing properties, and durable activity and did not penetrate polyethylene packaging films [187]. A sublimable gas-phase corrosion inhibitor formulation patented for temporary rust prevention has consisted of at least one polysubstituted pyrimidine, one monoalkylurea, and one C3-6 aminoalkyldiol, and optionally one benzotriazole with substituted or unsubstituted benzene ring. The above component can be co-mixed, or dispersed in water, or pre-mixed with solubilizer (such as alkylated phenol) composed of mineral oil or synthetic oil. The composition was efficient in protecting metals from atmospheric corrosion in sealed space [188]. The volatile ecofriendly RPO patented by Ping comprised of #32 base oil 50–80, octadecylamine 5–10, sodium sulfonates 3–5, barium sulfonate 1–4, sodium benzoate 2–5, polyvinyl alcohol 3–6, zinc butyl octyl dithiophosphate 1–3, polyether 2–6, sorbitan monooleate 1–5, octanoic acid tributylamine 2–4, lauryl alcohol 1–6, and amino silicone oil 2−5 wt. parts [189]. Shimizu and Yamada compared corrosion preventive properties of rust-inhibiting paper used for long-term transportation of cold-rolled steel sheets, and waterproofing paper used for domestic transportation. The rust-inhibiting paper comprised a structure of a VCI-impregnated craft paper successively laminated with polyethylene, and polyethylene cloth. The waterproofing paper, free from corrosion inhibitors, comprised a structure containing craft papers, adhesives, and polyethylene and/or oriented polypropylene, and are used with RPOs when applied on the steels. Accelerated corrosion testing clearly showed the superior performance of the rust-inhibiting paper to the waterproofing paper used with RPO. The study showed that when the rust-inhibiting paper and waterproofing paper have the same level of water-vapor permeability, better corrosion prevention was always observed for single use of rust-inhibiting paper than a combined use of waterproofing paper and RPO [190].

the loss of RPO [195]. Patents are available on automatic spraying devices. Kim disclosed a device comprising an upper electrode, a lower electrode and an anti-rust oil spraying assembly installed on the upper electrode. The spray assembly includes: an RPO storage unit, an oil spraying nozzle connected to the upper electrode through the storage unit and an oil supply pipe to spray oil onto the material; and a spray pump connected to the RPO supply pipe for discharging the oil to the injection nozzle. The device also comprised of an air jet body connected to the oil supply pipe to inject air when RPO is sprayed; an electric resistance sensor installed on an inner wall surface of the upper electrode to sense electrical resistance during welding; and a control unit [196]. Wang and Wang patented a kind of nail gun anti-rust oil automatic spraying device including sealable anti-rust oil tank, pipeline and oil injection valve [197]. Park invented an anti-rust oil injection device for a motor shaft for preventing corrosion during storage or transportation. The shimmer shaft can be easily inserted into the injection portion while preventing the anti-rust oil from leaking to the outside through the insertion portion into which the motor shaft is inserted [198]. Zheng patented a method and equipment for packaging liquefied gas storage tank where an RPO was injected to the liquefied gas storage tank in vacuum state so that the oil quickly atomizes and adheres to the surface of the liquefied gas storage tank [200]. An improved device for coating hot rolled steel strips (after pickling) with RPO prior to their wrapping into coils, by reducing loss of oil and controlling pollution of environment and fire hazard was disclosed by Prusty et al. [201]. Patents are also available on removal/cleaning agents of RPOs [202]. On comparing all these reported patents, it is evident that several efforts are currently going on to find novel oil, water or solvent-based RPs. Wax, resins and VCIs continue to be a preferred additive in formulations. Sulfonates remain to be the best organic corrosion inhibitor of choice. Sarcosine compounds were found to be employed in many formulations. Glycerol seems to be an attractive additive for green formulations. The following section describes the electrochemical studies reported (both old and recent works) on RPOs. Due to the similarity, important works published on lubricating oils are also incorporated. 4. Electrochemical studies The two primarily used evaluation methods for RPO coated metals are the humidity cabinet test (ASTM D1748) and the salt spray test (ASTM B117). Other testing methods employed include immersion testing (ASTM G 31-72), cast-iron chip test (ASTM D 4627-86), copper strip corrosion test (ASTM D130), and rust preventive test (ASTM D665). An RPs ability to displace water can be tested using military specification test (MIL-PRF-16173E), or water separability test (ASTM D1401). Most of these methods, however, require extended testing period and suffer from low reproducibility and cost factor. These methods often provide qualitative information and are not preferable for understanding the corrosion mechanism [203–205]. Electrochemical studies are important in this direction. Not many reported works are available on electrochemical analysis of RPOs. On the other hand, several studies are available on electrochemical evaluation of lubricating oils. Works reported on electrochemistry of metal/lubricant systems until 2000 are described by Zhu et al. [206]. The electrochemical techniques reported in evaluating RPOs includes corrosion potential measurement, cyclic voltammetry (CV), potentiodynamic polarisation (PDP), electrochemical impedance spectroscopy (EIS), and scanning electrochemical microscopy (SECM). Among these, EIS and PDP are the two mainly used methods. These techniques can offer ample evidence on the corrosion mechanism within a shorter duration. However, their application to RPO and lubricated systems are complicated mainly because of the low conductivity associated [207] which can be overcome to a significant extent by the use of nano/microelectrodes and supporting electrolytes. Zhu et al. have shown that the usage of nanoelectrodes enabled electrochemical measurements to

3.1.8. Patents on application methods / devices Several patents detailed pre-treatment approaches before application of RPOs. For example Shimoda patented a method with different pre-treatments prior to apply RPO that involves the following steps; (1) successively washing the stainless steel article with alkalis and then with HNO3 to remove cutting oils and baking hardened cutting oils, (2) hardening, (3) soaking in aqueous alkali hydroxide (0.05−1 g/L) for 12−20 h, and (4) applying RPO [191]. Hunag disclosed a method that includes (1) uncoiling steel coil, soaking into pretreating liquid at 30−40 °C for 3−5 min, and washing with deionized water, (2) drying at 100 °C, (3) coating RPO on double surfaces, and (4) coiling. The pretreating liquid comprised citric acid 10−20 wt.%, edetic acid 3–10, cetyltrimethylammonium bromide 2–8, Triton X-100 2–8, and water in balance. The RPO has consisted of Vaseline 69−80 wt.%, hexadecenylsuccinic acid 3–5, N-coco-β-aminopropionic acid 3–5, Tween20 4–6, and n-butanol 10–15 [192]. Iwamoto et al. invented a manufacturing method of steel product capable of suppressing occurrence of coating unevenness after a lubricating film is formed [193]. Several patents addressed anti-rust oil supply devices [194–201]. Jeon et al. patented such a device for supplying RPO for oiling steel sheets [194]. Park disclosed an RPO device and a rust preventive method by which it was possible to completely apply RPO to the inside and the outside of the object requiring rust prevention and to prevent 9

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Fig. 3. Impedance spectra for fresh and oxidized lubricating oils. Equivalent circuit model developed for the analysis of lubricant impedance spectra [213].

be performed in fairly low polarity fluids and, in some cases, with no supporting electrolytes. The technique, coupled with atomic force microscopy (AFM) friction measurement under potential control was shown to be a valuable tool [206]. There are several important works in this area prior to 1990s [206–210].

were examined to get the EIS parameters [216]. By pairing the in situ ATR–FTIR with EIS, much information on the diverse species in the lubricants were obtained as a function of the DC potential and oil degradation time. Li et al. employed six different formulations of RPs with different sulfonates that were dip-coated on steel panels, and dried at room temperature for 24 h. The samples details were as follows: RPB1 and RPB2 (barium sulfonate-based; RPB1 has higher oil content and RPB2 has lower oil content); RPS (sodium sulfonate-based formulation); RPC1 and RPC2 (calcium sulfonate-based, RPC1 has higher oil content and RPC2 has lower oil content); and RPA (amine sulfonate-based). The results of the study showed that for RPB1 coating, in the first day of exposure in 3 wt.% NaCl, the impedance response displayed a capacitive loop at high-frequency region (corresponds to the protective soft coating characteristics) and a low-frequency response (attributed to substrate/soft coating interfacial characteristics). After two days, the Nyquist plot showed only one smaller diameter loop, which signifies the change from an initial (water uptake stage) to the interface activation stage. Charge transfer resistance (Rct) decreased from 837781 to 16401 Ω. cm2 after two days whereas the constant phase element (CPE) increased from 5.10 × 10–7 to 6.56 × 10–5 S sn/cm2. Similarly, detailed investigation of all the above-mentioned systems were conducted and the order of the corrosion resistance was found to be: RPC1 > RPB2 > RPB1 > RPC2 > RPS > RPA [211]. Khemchandani et al. have shown that Nyquist plots of RPO coated systems exhibited only one depressed semicircle and its diameter was significantly higher for the optimum RPO (vegetable oil + optimised additives) coated sample, indicating that more densely packed monolayer of oil has formed on the metal surface [221]. Cesiulis et al. investigated the effect of chemical composition and physical properties of olive oils on their tribological performance and corrosion protection of steels. The results of contact angle and EIS showed that all the formed films of olive oils are porous; and hence the corrosion protection

4.1. Electrochemical impedance spectroscopy EIS has been used to characterize the performance of RP coated metals in different electrolytes. Both linear and non-linear EIS has been used in evaluating lubricating oil systems [208–216]. Studies are performed at higher temperatures [212,217] or in the presence of electrolytes [218–220] to overcome the poor conductivity of the lubricating oil. It has been shown that impedance diagram of a typical industrial lubricant can be divided into at least three regions (Fig. 3): bulk solution-influenced high-frequency range, medium frequency representing interface adsorption of surface-active additives, and low-frequency region related to diffusion and charge transfer processes. Fig. 3 shows Nyquist diagrams of both the fresh and the oxidized oils. Typically industrial lubricants lose their functionality through high temperatureinduced oxidative degradation and/or collective occurrence of different contaminants. However, this is not a major issue with RPOs in several applications. A proposed equivalent circuit model is also shown in the figure. The overall equivalent circuit model considered four processes: bulk relaxations, interface adsorption, bulk solution diffusion, and charge transfer at the electrode [213]. The authors in a subsequent work examined the application of higher harmonic non-linear electrochemical impedance spectroscopy (NLEIS) for analysis of industrial lubricants. NLEIS can deliver higher resolution than traditional linear impedance for distinguishing specific mechanisms governing electrode response [214]. Purushothaman et al. recorded EIS and IR spectra simultaneously at different DC potentials for various oil drain times and 10

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efficiency achieved was relatively low (57–75 %). The calculated works of adhesion of oil on steel were similar, but lower than that for water and that suggested that in the presence of water, the oils film can be displaced by water, resulting in corrosion [222]. Two- and four-electrode cell assemblies were employed for characterization studies of highly resistive non-aqueous automotive lubricants. It has been shown that more precise EIS studies can be carried out in two-electrode configuration with active shields where a single high-frequency arc and a complicated low-frequency impedance feature were observed [223]. These studies suggest that EIS as a non-destructive technique is perhaps the best electrochemical method for evaluating RPO coated metals.

micelles form on surface of surfactant, which surrounds water of thin corrosive solution; and subsequently concentration of micelles would reach a critical level associated with a potential decrease (Fig. 4d). The figure also shows the results of measurement of the PDP through thin corrosive solution on Fe-Cu-C sintered steels coated with RPOs. For the blank case (without oil) the corrosion current (icorr) obtained was 2.78 × 10−4 A/cm2 whereas RPO coated steels showed significantly lower icorr values: oil A: 4.64 × 10-7 A/cm2, oil B: 3.29 × 10-6 A/cm2 and oil C: 2.53 × 10-8 A/cm2, respectively. The result suggested that the RPO coating suppressed the anodic reaction significantly. The study showed that it is possible to evaluate RPO efficiency by the corrosion rate measurement [224,225]. Khemchandani et al. employed PDP techniques to explore corrosion behavior of biodegradable base oil-coated mild steel in 3.5 % NaCl. The results obtained by EIS and PDP measurements were found to agree with the conventional immersion and rust prevention tests (ASTM D665). The decrease of Tafel slopes of anodic and cathodic curves (due to the barrier film formation), shift of Ecorr to more negative values (base oils act as cathodic type inhibitors by restricting the oxygen diffusion process) and decrease of icorr suggested improved corrosion resistance of RPO coated steel [221]. To overcome the drawbacks (difficult to select the potential region and times of cyclic scanning) of repeated cyclic voltammetry (RCV) in evaluating the properties of RPOs, a phases repeated cyclic voltammetry (PRCV), was reported by Yao et al. Comparison of the results of PRCV, RCV, and humidity cabinet test showed that the former method is better than the other two in simplicity, convenience, and reliability. Four RPOs, 1) oil only, 2) oil + petroleum barium sulfonate, 3) oil + zinc naphthenate, 4) oil + polyisobutylene were investigated. A three-electrode electrochemical cell (electrode coated with RPO, auxiliary platinum electrode, and saturated calomel electrode connected by a salt bridge) was employed for both RCV and PRCV measurements (NaCl solution, room temperature). RCV experiments were performed for 8 cycles from –0.6 to 0 V at a scanning rate of 50 mV/s. The rust preventive ability of RPO was determined based on the number of cyclic scanning and the maximum current densities recorded. For PRCV, the potential region was Δ 0.3 V and the scanning started from –0.3 V. The continuous potential regions selected were (–0.3 ∼ 0 V), (0 ∼ 0.3 V), (0.3 ∼ 0.6 V), (0.6 ∼ 0.9 V) etc. With each potential region,

4.2. Potentiodynamic polarisation and cyclic voltammetry Electrochemical techniques, like PDP, can be used to qualitatively compare the corrosive behaviour of a variety of metals in solutions containing compounds commonly added to RPOs or lubricants [207] and hence can be used to identify concentration ranges where they provide most corrosion inhibition. However, employing PDP or CV to investigate corrosion behavior of RPO/lubricant coated metals in an aggressive electrolyte may lead to erroneous results. Iwashima et al. developed a method to quantitatively assess the rust prevention capability of RPOs by polarization curves measurement through a thin corrosive solution on Fe-Cu-C sintered steels coated with RPOs. The samples were prepared by dip-coating for 15 min, followed by keeping in a vacuum desiccator to remove surface bubbles. Three kinds of test oils; namely oil-A (conventional RPO), oil-B (without rust inhibitor and oil film modifiers used in oil-A) and oil-C (containing different oil film modifiers from that of oil-A) were used. A 0.35 mass % NaCl aqueous solution at pH 5.76 was used as the electrolyte. A schematic representation of the polarization set-up used is shown in Fig. 4. Polarization curves were recorded after corrosion potential stabilization (30 min). Fig. 4a shows the result of corrosion potential measured as a function of time for the sample coated with oil-A. During 0–140 ks, severe potential fluctuations were observed, and that is suggested to be due to the repetitive formation and destruction of adsorbed layer of surfactants at the interface. At initial stages, surfactant in the RPO gradually aligned to the oil/water interface and their concentration increased with time (Fig. 4b and c). After an increase of the potential,

Fig. 4. (a) Corrosion potential of sintered Fe-Cu-C alloy measured with oil A in thin corrosive solution and schematic diagram of the formation of micelle in the first (b), second (c) and (d) third stage. (Right) Schematic diagram of apparatus used for the polarization curve measurement. The corresponding polarisation curves of the coated alloy measured through thin corrosive solution in 0.35 mass % NaCl at 298 K is also shown [224]. 11

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Fig. 5. Scanning Kelvin Probe potential profiles of oiled sample A, B and C in salt spray test after different durations. Corresponding photographs of oiled samples after salt spray test of 2, 19, and 43 h are also shown (SKP study conducted on the marked area; 8 × 4 mm) [10].

the scanning performed twice until a fairly large current density was obtained. The rust preventive ability of RPO was deduced by comparing the number of potential regions and the maximum current density recorded in the same potential region. Results of RCV studies shown that the voltammograms of electrodes coated with the four kinds of RPOs were different. The maximum current density recorded after 8 cycles was the largest for the 1# oil (2.4 mA/cm2), while that of 4# oil was the smallest (0.2 mA/cm2). The difference between current densities of electrodes coated with 2# oil (1.3 mA/cm2) and 3# oil (1.2 mA/cm2) was not obvious. The rust preventive ability was in the order 1# < 2# < 3# < 4#. Results of PRCV studies showed that no current density has appeared in the two cyclic scans of 1# oil-coated electrode in the potential region (–0.3 V ∼ 0 V), but the current density was ∼12 mA/cm2 after the two scans in the (0.3 ∼ 0.6 V) region. No obvious current density responded for the electrode coated with 2# oil within the potential regions (–0.3 ∼ 0 V), (0 ∼ 0.3 V), and (0.3 ∼ 0.6 V); whereas the maximum current density was ∼12 mA/cm2 after scanning in (0.6 ∼ 0.9 V) region. No current density appeared for 3# oil-coated electrode in (–0.3 ∼ 0 V), (0 ∼ 0.3 V), (0.3 ∼ 0.6 V), (0.6 ∼ 0.9 V) regions; however, the maximum current density was > 12 mA/ cm2 after two scans in (0.9 ∼ 1.2 V) region. There was no current density for 4# oil-coated electrode in regions (–0.3 ∼ 0 V), (0 ∼ 0.3 V), (0.3 ∼ 0.6 V), (0.6 ∼ 0.9 V), and (0.9 ∼ 1.2 V) until the current density responded in (1.2 ∼ 1.5 V). The results also suggested that the order of the rust preventive ability was 1# < 2# < 3# < 4#, which were further supported by humidity cabinet studies [226]. Sing et al. performed a CV test for 9 cycles on RPO coated steels. Three RPOs with different viscosities (A, B and C oils with viscosities of 345, 226, and 35 mPa.s respectively) and total base number (11.5, 2.1, 6.9 mg KOH/g) were used. The CVs recorded for all the samples have a similar nature, but their current density values were different, and that increased with successive cycles. A low peak current density recorded for sample A (about 3.5 times lower than that of samples B and C) corresponds to the better insulating character of the oil film [10].

measuring the electrochemical properties. The results suggested that when compared with an oil-coated wire-beam electrode, the electrochemical states on an uncoated electrode were homogeneous, and the former is suitable for examining electrochemical inhomogeneity of RPO coatings [219]. It has been showed that the corrosion potential distribution on an oil-coated wire-beam electrode follows a discontinuous binomial probability distribution, and the distribution changes considerably on adding oil-soluble inhibitor to the base oil [231]. Zhong et al. explored three types of lubricants, differing in their inhibitor content: sodium silicate, triethanolamine and sodium nitrite, by using a wire beam electrode in NaCl solution. Two criteria were used: one was whether or not the lubricant might change the distribution of corrosion potentials of an oil-coated electrode, the other was whether or not the lubricant was beneficial to the homogeneous distribution of corrosion potentials of an oil-coated electrode. The results showed that sodium nitrite was the most effective lubricant inhibitor [14]. More details can be found elsewhere [14,219,227–231]. 4.4. SECM studies One report is available on SECM studies on RPOs. Singh and Rani coated three RPOs (A–C) on bearing steels, and their corrosion performance was studied by scanning Kelvin probe (SKP) microscopy. SKP is a non‐contact method that measures Volta potential difference between the sample surface and Kelvin tip. The results, as expected displayed that the corrosion potential of all the oil-coated samples decreased significantly attributed to the decrease in oxygen reduction reaction. The SKP studies were carried out before (0 h) and after 2, 19, and 43 h of salt spray test (SST) (Fig. 5). The results showed that the corrosion potential of sample A increased during SST exposure (−165 mV at 0 h to 232 mV after 43 h SST). As no rust was observed on experimental area (see photographs below), it was inferred that increase in potential was due to the surface passivation; and that was correlated to the total base number of oil A (11.5 mg KOH/g). The corrosion map of sample B showed a small negative shift of potential after 2 h suggesting the onset of active corrosion and that was associated with the low total base number and high oxidation peak and not to a passive film formation. A sharp potential dip happened after 19 h due to corrosion and after that, an increase at 43 h due to defect blockage by corrosion products. Similarly the corrosion potential map of sample C showed a large potential drop at 2 h due to active corrosion (no passive characteristics). The low corrosion resistance of sample C was attributed to the high dosing of Mg and Zn in formulation which acts as active corrosion sites during SST. Only a thin film was formed

4.3. Wire beam electrode method Reproducibility and reliability of electrochemical measurements of coated electrodes may be improved by using segmented electrodes. A wire beam electrode composed of several mild steel wire sensors (∼ 100 numbers having a diameter of less than 1 mm imbedded in an epoxy resin at ∼ 2 mm from each other) was developed and used to study the anti-corrosion performance of lubricants [14,227–230]. Each wire acts both as the substrate for the coating and as a sensor for 12

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due to low viscosity and specific gravity of oil C resulting in inadequate barrier protection. Oil A possesses low oxidation peak, low total acid number, and high total base number that signify higher corrosion resistance. The SKP results were supported by conventional salt spray and condensation water tests [10].

in order to enable easier penetration into deep interiors of a small bearing), RUSTOP 276 (solvent cut back type, useful under severe salty atmosphere), RUSTOP 281 (RPO for steel rolling industry using electrostatic oilers), RUSTOP 282 (oil-based, having a higher film thickness), RUSTOP 285, 286, 287, S (oil-based rust preventives; lubricating oils containing soluble corrosion inhibitors), RUSTOP 387, 388 (greasy film type rust preventives, for hot dipping), and RUSTOP 389 (semi fluid type recommended by certain forging industries) [236]. Dry Coat™ RP is a water-based safe and environmentally friendly liquid RP for ferrous metals. It is projected as a user-friendly replacement for solvent and oil-based RPs and easy to apply via spray or dip. According to the manufacturer, at ambient conditions, this coating will dry-to-touch shortly and does not attract dirt or dust. It can be removed easily, with a mild detergent or cleaner [237]. Petrofer has a series of dewatering fluids that is built following the evaporation of the solvent. Coatings are available in both oily and slightly waxy type. The products include ISOTECT WSD 212 (fast evaporating dewatering fluid that leaves a somewhat waxy film), ISOTECT OSD 213 (quickly evaporating and leaves an oily film), ISOTECT WSD 310 (medium evaporating and leaves a slightly waxy film), and ISOTECT OSD 311 (medium evaporating and leaves an oily film) [238]. DAUBERT NOX-RUSTⓇ RPOs are based on the contact corrosion inhibition technology and are formulated to provide long-term protection to vehicle bodies [239]. FUCHS has an Anitocorit CPX waxbased product for long term corrosion protection [240]. Cortec Corporation has marketed different water-based rust preventatives powered by Nano VCI [241]. Details of products of several other companies such as BECHEM, Shell, Croda etc. are available in their respective websites.

5. Semiconducting properties It was suggested that there exist ionic and electronic films in RPO coatings, which lead to inhomogeneous corrosion potential distribution on the metal surface and discontinuous binomial distribution [227–231]. Zhong et al. studied semiconducting behavior of a temporary oil coating on AISI 304 stainless steel in 5 % Na2SO4 solution by potential-capacitance method and Mott-Schottky analysis. The coating behaved as a semiconductor during degradation. At the early immersion period, an anodic process was predominant at the metal/oilcoating interface, and the coating behaves as a p-type semiconductor. With increase of immersion time, a cathodic process becomes dominant, and the oil coating transformed from p-type to n-type. As the period of immersion increased, the oil coating may change from electronic to ionic as ionic diffusion in the coating becomes high [232]. The authors in subsequent works revealed that with increasing immersion time, the capacitance of the space charge layer of the oil coating augmented abruptly and that is attributed to the chloride ion diffusion and infiltration into the oil film, increasing the number of charge carriers. The results concluded that the oil coating considerably blocked the anodic processes. During its degradation, the coating becomes increasingly semiconducting and the ionic penetration noticeably influenced the semiconducting behaviour [233,234]. 6. RP commercial products

7. Conclusion and future perspectives

Several companies are developing commercial RPs. Here, we have provided a short description of commercial products of a few companies so that the reader gets an idea about the nature and types of industrial products currently available. Table 1 shows representative examples where various RPs of ‘Zerust’ are provided (Table 1) [235]. HP lubricants have several RP grades selectively formulated for different applications and offer the following advantages: non–staining, compatible with lubricating oils, easily applied by brush, spray or swa, easily removable by solvent and economical. The products include HP RUSTOP 172 (for protection of electric resistance welded tubes), HP RUSTOP 173, 184 (solvent cut back type, useful under moderate conditions and for interim protection; Rustop 173 has water displacement characteristics, and Rustop 184 is a light moderate duty rust preventive giving nondrying film), RUSTOP 173 DW (water displacing, nonstaining dewatering fluid), RUSTOP 175 (solvent-based rust preventive oil that forms an acid fume resistant film), RUSTOP 274 (a premium rust preventive oil used in final packing stage in critical applications such as in ball bearings; in addition to water displacing property, it has an additional finger print neutralizing property), RUSTOP 275 (a premium low viscosity RPO specially designed for usage in small bearings

In spite of the high significance of temporary rust preventives, a comprehensive review article highlighting the research advancements is lacking in the current literature. Here, we analysed the topic and presented in the most accessible way. Each of the different types of rust preventives as discussed has its advantages and disadvantages. The choice depends on the requirement and the nature and the extent of protection required. Most of the reported information in this area deals with oil-based rust-preventives. Solvent-based systems, on the other hand, are not attractive to environmental concerns. Water-based systems are the favoured choice in terms of environmental friendliness, and biodegradability. Several works addressed resin-type strippable coatings. Wax remained to be a major component in rust preventive formulations. Many systems employed VCI as a component in the formulation. Several reported works employed many of these components together in a single formulation. Highly efficient rust preventives can be developed through suitable optimization of the type and quantity of the components in the formulation. A novel rust preventive formulation is expected to be environmentally friendly with no odour, high safety, long-term rust prevention, and broad application prospect. In the subsequent cleaning

Table 1 Industry products from Zerust®/Excor®. Technical data is available on the company website [235]. Product name

Product information

Axxanol 33 Axxanol 33CD Axxanol 34CD Axxanol 750 Axxanol Z-Maxx Series Axxanol 46-BIO Axxanol Spray-G Axxatec 77C, 80C Axxatec 85F, 87M

Ready to use rust preventative oil liquid. Light amber liquid coating results in a thin oily film. Solvent-based corrosion preventative liquid. Amber liquid dries to an invisible and non-tacky coating. Ready to use solvent-based rust preventative. Amber liquid dries to a thin oily film that is near dry-to-touch. Ready to use oil-based VCI rust preventative liquid. Light amber liquid coating results in a thin, oily, clear, water barrier film. Corrosion inhibitor lubricant greases. Bio-based, sprayable corrosion inhibitor preservative in the form of light lubricant oil. Solvent-based corrosion inhibitor sprayable grease. Water-based spray-on, in-line production or dipping applications. Water-based rust preventative with VCls to protect ferrous metals.

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process, the film is expected to be easier to clean and remove, and may not cause excessive pollution and waste cleaning. More than 90 % of the recently reported works on rust preventives appeared as patents. Even though it was difficult to classify the patents explicitly, a general classification is made for easy readability and the information within each patent briefed. A number of recent patents addressed different application methods and devices related to rust preventive oils. Several noteworthy works in this area are available in literature prior to 1980s. Only a few works addressed electrochemical evaluation of RPOs. On the other hand, several reports are available with electrochemical studies in industrial lubricant systems. Different electrochemical techniques such as cyclic voltammetry, electrochemical impedance spectroscopy, scanning electrochemical microscopy and scanning tunneling microscopy need to be explored further with modified experimental systems suitable for RPO coated metals. More research works need to be addressed to develop highly efficient and commercially relevant RPOs with (i) enhanced active protection (by using controlled release inhibitor capsules) (ii) better biodegradability and (iii) enhanced oxidation and UV resistance. The first aspect is important as inhibited oil may slowly lose its protective value through the gradual leaching of rust preventive agents in the film. The concept of on-demand release smart coatings thus can be extended to RPOs [242]. Further studies on novel multifunctional RPO formulations suitable for electrostatic spraying is required. More studies on modified vegetable oils, their esters and polymeric materials as a substitute to their petroleum-derived counterparts can be attractive in terms of cost, availability and biodegradability [243]. Use of biodegradable corrosion inhibitors is highly preferred. Superhydrophobization of metallic surfaces (high water contact angle > 150°) for corrosion protection (due to water-repelling capability) has attracted significant recent research attention [244–246]. Superhydrophobic surface modifications/coatings are particularly suitable for temporary corrosion protection for specific applications; for example for rendering temporary corrosion protection in hardware sitting on a launch pad for 30–45 days before a satellite launch [247]. Wax is a major component in many recently reported superhydrophobic coatings in different applications [248,249]. The concept of superhydrophobization needs to be further explored in this direction for achieving superior temporary corrosion protection. The trend shows that environmentally friendly VCI is a viable method for interim corrosion prevention, especially as a component in RPO formulations [18]. By incorporating VCIs in naturally derived solvents/oils, best corrosion protection can be attained yielding ecofriendly biodegradable products. The concept like VCI protection with low-volatile organic inhibitors in specialized chambers at elevated temperatures [250,251] can be further explored. By this review, we have tried to comprehensively present all the information available under rust preventives with a view to enhance future R & D in this area for its high industrial applicability. The information presented is useful for both academicians and industrialists.

[5] Temporary corrosion preventives, Anti-Corros. Method Mater. 14 (1967) 17–19, https://doi.org/10.1108/eb005199). [6] T.N. Tate, E.W. Beale, Temporary protectives, Shreir’s corrosion Vol. 4 Elsevier Science, Amsterdam, 2010, pp. 2678–2682, https://doi.org/10.1016/B978044452787-5.00146-3 ISBN: 978-0-444-52787-5. [7] Guide to Temporary Corrosion Protectives, National Corrosion Service, NPL, 2003, https://www.coralpack.com/PDF/Corrosion/temporary_corrosion_protectives. pdf. [8] G. Moran, B. Faber, N. McGuire, Fundamentals of rust preventives used for temporary corrosion protection, Tribol. Lubric. Technol. 72 (2016) 28–34. [9] J. Banik, P. Kuhn, M. Ruthenberg, A. Schrooten, S. Sikora, Temporary Corrosion Protection Layer, WO 2018060082 A1 20180405 (2018). [10] A.K. Singh, N. Rani, Scanning Kelvin probe study of steel/oil interfaces for corrosion evaluation, Mater. Corros. 70 (2019) 1162–1170. 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