How To Loosen Tight Rusted Bolts without Heat, Using Mild Acidic Penetrating Liquids and Rust Dissolvers | Friction and Rusting Explained
- Mechanics of Bolted Joints
- Mechanics of Moving Joints
- Conditions which Increase Friction in Bolted Joints Causing Tight and Stuck Bolts
- What is the Difference between a Stuck and Seized Bolt?
- Why Penetrating Fluids are Better than Heating in Loosening Stuck Bolts?
- Metal Corrosion and the Rusting / Oxidation of Iron Explained
- Rust Removal Methods
After some time, bolted metal joints or connections in structural steelwork, machinery, equipment and appliances will prove hard to unfasten (unscrew, loosen or unbolt) as the bolt and nut gets stuck, making it impossible to turn with a torque wrench. Many people will encounter this problem after the long-term use of a machine or appliance, especially if the machine was not maintained properly or lubricated as needed. However, depending on the item, its built state and usage purposes, some joints don’t need lubrication but friction for them to work and hold firmly, for example this is the case in many structural engineering works such as bolted base plates and anchor bolts. Both static and moving joints can have their fasteners or connecting parts stuck for a variety of reasons that are going to be discussed below. In building, many structures have static bolted joints, for example a structural steel portal frame, steel truss, steel floor joists, steel framework ceiling, recessed screw manhole covers, steel gate and palisade fence. In mechanical engineering, most structures are often characterized by moving joints. Examples include vehicles, conveyer belts, grinding mills, escalators, elevators, wind turbine, cranes, hydraulic turbines and a wide range of other industrial equipment and machinery.
Mechanics of Bolted Joints
Before we look at how or why bolts get stuck, we should look at the mechanics of bolted joints to understand the conditions and forces working together to get the fasteners stuck, jammed or freezed. First, we have to differentiate the types of joints available in engineering. As mentioned previously, we have static and moving joints. Bolted joints are static joints. Static joints are non-moving friction joints known in structural engineering as slip-critical joints. The parts being connected together are static. In this type of joint, a greater amount of friction is required and desirable in producing clamp loads and bolt tension during tightening to provide stability and prevent slippage between the connected parts or surfaces.
In a bolted joint or connection, you can easily recognize three parts, the bolt, nut and the clamped part. Washers may also be needed before the nut is placed and tightened with a torque wrench. The bolt is a narrow, elongated and threaded solid metal cylinder. To fix the assembly parts together, a hole is drilled through the clamped part to allow for insertion of the bolt, with the other end secured with a nut. Alternatively, in other settings and applications, the bolt can be embedded in wet concrete with the thread protruding above the surface to allow for putting on the steel base plate, nut and washer. In this case, the clamped parts are the base plate and hardened concrete. In the case of cladding panels fixed to structural steel framework, the bolt will go through the panel and steel purlins. In this fixture, the clamped parts are the panel and steel purlin.
To understand the forces acting on the bolted joint, refer to the diagram below:
T = Tensile Force
C = Compressive Force
B1 = Torque (turning force) applied on bolt cap. Applied torque includes pitch torque responsible for bolt tensioning + torque required to overcome friction.
B2 = Torque (turning force) applied on nut. Applied torque includes pitch torque responsible for bolt tensioning + torque required to overcome friction.
TF = Thread torque – frictional torque between threads (reacting to/opposing the applied torque B1 or B2).
TB = Bearing surface torque – frictional torque between underside of bolt head and clamped surface (reacting to/opposing the applied torque B1 or B2 under)
In physics, Newton’s 3rd Law of Motion says for “every action, there is an equal and opposite reaction”, therefore:
Applied torque = Pitch torque + Bearing surface friction torque + Thread friction torque
So what is the relationship between the forces mentioned above and how do they work to tighten the bolt as well as untighten/loosen it?
When you use a torque wrench to turn either the bolt head or nut, the tightening torque applied on either of these positions works to stretch the bolt in the longitudinal plane, and at the same time compressing the clamped part between the bolt head and nut (clamp load). The pitch torque responsible for bolt tensioning is directly proportional to the compressive clamp load, connected by the equation:
Pitch Torque=Thread Pitch x Clamp Load/2π
Thread Pitch is defined as the distance between threads on a bolt measured in millimetres.
The applied torque (input torque) is also directly proportional to the compressive clamp load, connected by the equation:
where T= Applied Torque,
K=Nut Factor (Values 0.03 to 0.35),
D=Nominal Bolt Diameter and
F=Desired Clamp Load.
Part of the applied torque is used to overcome friction in the bolt threads as well as on the underside of the bolt head and nut. A larger percentage of the tightening torque, about 90% is energy lost (in the form of heat dissipation) while overcoming friction and the rest (10%) is elastic energy used to stretch the bolt and compress the clamped part in the process. Energy lost while overcoming friction is split 50:40 between the bearing surfaces and bolt thread.
Relationship Between Torque and Friction:
Friction is a force that resists motion between two surfaces in contact moving against each other. It is represented by the equation:
F= Force of Friction,
μ=Coefficient of Friction,
Re-arranging the equation, μ=F/N
The coefficient of friction is always a relative value between two surfaces, fluids or materials in contact and moving past each other. Thus, the coefficient of friction will vary between various surfaces or materials.
As you may well be aware, friction produces heat (wasted energy) when a moving layer transfers kinetic energy to the stationery layer leading to an increase in temperature, or when kinetic energy is transferred between sliding interfaces, but what causes friction?
Friction can’t be completely eliminated but it can be reduced, it exists even in a vacuum (outer space and the moon). As long as there are two surfaces in contact, rubbing or sliding over each other, friction will exist. It occurs due to the fact that there is no material with a completely flat and smooth surface at a microscopic or atomic scale. The smoothest surfaces in your household like mirrors, glass, silk, marble, stainless steel and polished porcelain may look and feel smooth to the touch, but they are actually rough at an atomic scale. In tribology, the roughness and topography of a material surface at a microscopic scale is known as surface asperity. Asperities are the protrusions on an uneven surface. Under frictional force and compressive clamp loads, when one surface slides over the other, the asperities rub over each other generating heat, eventually leading to plastic and elastic deformation whereby the asperities are crushed under load, shearing off, and in extreme cases such as metal galling, fusing and welding of the asperities takes place under increased heat, creating metallic bonds which are stronger than the base metals.
According to our previous equations, we said that both the applied torque and pitch torque are directionally proportional to the clamp load or tension in the bolt. We also said that about 90% of the applied torque is lost in the form of heat dissipation while overcoming friction. This means that if we were to reduce friction in the bolt threads and bearing surfaces under the bolt head and nut, more tightening torque will be converted into clamp load, but since a bolted joint requires increased friction to lock and hold it firmly in place, reducing friction between the threads and bearing surfaces will loosen/untighten the bolt and nut, resulting in a weak joint. Joint slip and self loosening are undesirable, not only in structural engineering, but also in other fields such as automotive and petrochemical engineering. Reducing friction may be undesirable in bolted joints, but it is a requirement in moving joints.
Mechanics of Moving Joints
Unlike a static joint, a moving joint has at least one of the connecting parts or surfaces moving, rotating, sliding, brushing, pushing, pulling or vibrating, and making contact with the other part during the motion. In moving mechanical joints, less friction and more lubrication is required to achieve optimum performance and prevent the joints from freezing or getting stuck. Examples of dynamic mechanical joints include ball bearing joints, rod and piston, swivel joints, socket & spigot joints, pin joints, hinge joints, knuckle joints and turnbuckle joints.
Galling (unintended fusion and cold welding of metal surfaces in industrial and engineering applications leading to freezed joints) is a common phenomenon in moving joints. It occurs when there is increased friction between sliding metal interfaces due to lack of lubrication and other factors such as material properties. High ductility, low surface hardness (resistance to indentation) and poor surface stability provide a conducive environment for galling. Surface texture and finish also play a role in galling in that extremely smooth surfaces increase contact between them, causing molecular adhesion and attraction which leads to galling. Rough surfaces also promote galling caused by classical interlocking, shearing off, fusion and welding of asperities. An intermediate surface texture or finish is therefore required to prevent galling. It’s very important to note that in the case whereby lubrication is required between interfaces, the surfaces should be adequately rough to hold lubricant in the gaps between the asperities since a smooth surface has poor fluid retention properties.
Conditions which Increase Friction in Bolted Joints Causing Tight and Stuck Bolts
By now, you should know that friction is essential in locking and holding bolted joints firmly in place to provide stability to the assembly. However, increased friction in the bolt threads and bearing surfaces caused by a number of factors such as environmental conditions can lead to problems which make it difficult to untighten or loosen the bolt during disassembly of the equipment, structure, machine or appliance.
A newly installed joint secured with a new bolt and nut will yield to untightening/loosening with a torque wrench soon after tightening using only 70 to 90% of the initial tightening torque. However, as time passes by, temperature changes, buildup of dirt, debris and rust caused by corrosion of iron, steel and other iron alloys will increase friction in the threads which requires an untightening torque much greater than the initial tightening torque. Actually, changes in friction affecting untightening of bolts and nuts can be observed as early as 12 hours after tightening the joint, and in this short space of time, temperature, initial bolt tightening speed and pressure will more likely be responsible for increase in friction. The effects of accumulation of rust, dirt and debris particles in the threads will be realized a few weeks or months later, causing a further increase in friction, resulting in stuck bolts.
What is the Difference between a Stuck and Seized Bolt?
For differentiation purposes, a stuck bolt is a tight bolt which cannot be untightened or loosened with a torque wrench, but this bolt hasn’t undergone galling or serious plastic/elastic deformation yet, giving it a high chance of being loosened with penetrating fluids and rust dissolvers.
A seized bolt has undergone freezing and galling (fusion and welding of thread interfaces), which makes it impossible to remove without breaking the bolt into pieces. A seized bolt will be rendered useless and un-reusable after its removal, but a stuck bolt may be re-used due to its preserved shape provided that it’s not badly corroded.
Why Penetrating Fluids are Better than Heating in Loosening Stuck Bolts?
Heating a stuck bolt should be a last resort when penetrating oils have failed to loosen the bolt. The problem with heating is that it’s often hard to control precision i.e. containing and concentrating heat on the exact area that you want to heat without heating up the surrounding areas. Heating up the nut/bolt with a blow torch or heat gun will most likely conduct the heat to the clamped part, damaging the material by thermal deformation. You also have to ask, does heat produce changes in the size (diameter) of the metal plate hole?
By applying heat on the joint, the hole on the metal plate (clamped part) as well as the bolt will expand, increasing in diameter. As the temperature drops, the hole and bolt diameter will contract, shrinking in size. Provided that the metal plate is not heated beyond the yield point (elastic limit), the hole will shrink back to its original size. Overheating may permanently deform the metal plate and alter the hole diameter once the yield point is exceeded.
Metal Corrosion and the Rusting / Oxidation of Iron Explained
Rusting is a type of metal corrosion which occurs in iron and its alloys such as steel when exposed to oxygen and water. Under normal earth conditions, rusting will not take place when either of these elements is absent. However, according to the chemical equation:
Iron + Oxygen = Iron(lll)oxide
4Fe + 3O2 = 2Fe2O3
Iron(lll)oxide is the chemical name for rust, so theoretically rusting can also take place in the absence of water when oxygen is available, but it will take an eternity for the rust to form. When water or moisture is added to the mix, the chemical reaction between iron and oxygen is accelerated such that you will see the formation of rust with a week or two. It can be seen that water acts as a catalyst which acts to speed up the rate of reaction which causes the corrosion of iron. Many more mediums can act as catalysts for the rusting process, this includes salt water, acids, saline humidity, aqueous solutions, galvanic reactions between surfaces of different electrode potentials and high temperatures. Thus, when water is added to the equation, it will look like this:
Iron + Oxygen + Water = Hydrated Iron(lll)oxide,
4Fe + 3O2 + H2O = 2Fe2O3.(H2O)
Hydrated Iron(lll)oxide as the name suggests contains a variable amount of water (nH2O) whose percentage depends on the amount of moisture in the environment. The variable amount of water (n) can be shown in the equation below, where (n) represents the number of moles / quantity:
4Fe + 3O2 + nH2O = 2Fe2O3.(nH2O)
The reaction in the water also produces positive iron(ll) ions in an aqueous state (Fe2+) . These positive iron(ll) ions react with aqueous negative hydroxide ions released from water (2OH -1) to form iron(ll)hydroxide Fe(OH)2, which is an insoluble compound.
Iron(ll)hydroxide Fe(OH)2 reacts further with water and oxygen to form Iron(lll)hydroxide:
Iron(ll)hydroxide + water + oxygen = Iron(lll) hydroxide
4Fe(OH)2 + 2H2O + O2 = 4Fe(OH)3
What You Must Know:
We mentioned three terms so far:
- hydrated Iron(lll)oxide
All these are rust in different chemical states and formula. Iron(lll)hydroxide which is also known as Iron(lll) oxide-hydroxide is saturated with water, along with its hydrate (hydrated Iron(lll)oxide), while pure Iron(lll)oxide has no water molecules. Well known as a product of oxidation, rust is also a hydrate (product of the hydration reaction) leaving dark orange, brown or red crystalline particles on the corroded surface once the water of hydration or crystallization is lost. It is insoluble in water, but soluble in several mild and strong acids as well as sugar solutions.
Many naturally occurring minerals, metals and metal ores exist either as crystalline or polycrystalline solids (solids with highly ordered atomic structure), which is a more stable state than an amorphous state.
Manufactured metals and alloys including those that have been produced by volcanic activity are amorphous solids (non-crystalline solids with irregular atomic structure).
According to the law of equilibrium, when disturbed or made unstable, stable systems have a tendency of returning to their state of stable equilibrium. Corrosion can be explained as a reaction caused by the metal and its environment attempting to reach thermodynamic equilibrium. Thus, a manufactured metal in its less stable (high energy) amorphous state is inevitably bound to disintegrate (corrode) and return to its more stable (low energy) crystalline state (rust), which is its native naturally occurring state.
Why is Corrosion of Iron/Steel More of a Concern than other Metals?
Oxidation does not only affect iron and steel alloys, but it also affects other metals such as aluminium, nickel, chromium and magnesium. When exposed to corrosion, these metals will produce oxide particles on the surface (similar to the rusting process), but the difference is that this oxide layer is a not a loose and flaky layer like rust. Instead, it functions as a tight protective barrier that is highly resistant to penetration and degradation. As the oxide film builds up over time covering the entire surface, no further corrosion will take place.
The problem with iron and steel alloys is that the oxide layer (rust) is not a protective coating, so corrosion will keep on attacking the newly exposed metal surface until the metal is reduced to a red or brown powder.
How To Loosen Tight Rusted Bolts with Mild Acidic Penetrating Liquids and Rust Dissolvers
To loosen tight, rusted and stuck bolts on your equipment, appliance or machine, there are several penetrating oils and mild acids that you can use using a method known as metal pickling. Most of these rust dissolvers are found in one form or another as household cleaning agents or non-toxic mild acids such as white vinegar and pure citric acid extracted from fruits such as oranges, lemons, limes, naartjes and grapefruits.
Most penetrating oils are commercial and industrial products manufactured under different brand names, and widely used in the automotive, manufacturing, engineering, marine, petrochemical, plumbing and drainage applications. A good penetrating oil does not only loosen the rust, grime, dirt and mating bonds, but it also penetrates deep into the tiniest spaces between fittings and joints.
Depending on the severity of the situation i.e. buildup of rust, amount of friction and strength of adhesive mating bonds between the joints and threads, you might need to wait longer after applying the rust dissolver (or pickling liquid) before the bolt begins to loosen. Use a bottle with a small nozzle to apply the liquid acid or penetrating oil. Spray the solution directly on top of the nut and bolt, as well as on the joints.
Rust will begin to loosen after 10 to 12 hours depending on the type of rust dissolver or penetrating oil you are using. For tough situations, you might need to wait days or even weeks before the bolt loosens. To hasten the loosening, keep applying the dissolver at regular intervals once the penetrating fluid has soaked in.
Rust Removal Methods
Non-Toxic Mild Organic Acids
White vinegar also known as Acetic acid.
Citric acid obtained from citrus fruits (Oranges, lemons, naartjes, limes, grapefruits)
Levulinic acid – Obtained from hydrolysis of sugars, cellulose and starch, it used in production of herbicides, cosmetics, perfumes, fragrances, resins, animal fodder, anti-freeze and solvents. It’s highly soluble in water, available as a colourless liquid or white powder.
Glycolic acid – Also known as hydroacetic acid, it is extracted from sugar plants and crops such as pineapple, sugarcane, sweet sorghum and beets. However, it can also be chemically produced from fossils and synthesized from microbe enzymatic catabolism of glycolonitrile and ethylene glycol. Glycolic acid is used in various skincare products to treat ailments, as a textile dye, food preservative, flavouring agent, in household cleaning products and other applications. It’s available as a transparent liquid or white crystal powder.
Gluconic acid – Obtained from the oxidation of a specific glucose carbon (D-glucose) via bacterial fermentation, enzymatic hydrolysis of glucose-D with glucose oxidase secreted by fungi and insects and chemical hydrolysis of starch using aqueous bromine, the acid is also found naturally in honey, wine, fruits, rice, beer, vinegar and grape juice. The acid is used in food additives, cosmetics, dyes, ink, home cleaning agents, medical treatments of acid burns, hypocalcemia, tissue necrosis and other issues. It’s available as a white crystal powder, granules or dark yellow liquid.
Tartaric acid – Also available as a powder known as cream of tartar which is a by-product of wine production, this organic acid is extracted from plants such as grapes, tamarinds, apples, avocados, apricots, beets, sugarcane, mangoes, cherries, peach, pineapple, strawberries, pear and papaya. Tartaric acid has many uses in the food industry, for example in baking. Liquid tartaric acid is clear and colorless, the powdered form has white crystals.
Toxic Mild Inorganic Acids
Phosphoric acid – This inorganic acid is corrosive and hazardous, capable of irritating, burning, cracking and damaging human tissues both inside and outside the body, so use it with care. Dilute it to less than 10% concentration if you want to prevent serious health complications. Avoid ingestion, inhalation, skin contact and fumes. However, due to its effectiveness, it’s widely used as a commercial cleaning solution for removing stains in sinks, bathtubs, tiles and toilet bowls. It’s a very effective rust dissolver. The liquid and solid form are both clear and colourless, and the liquid is a highly viscous 75% to 85% aqueous solution.
Toxic Strong Acids
Strong acids are very effective in dissolving rust, acting much more quickly and aggressively than mild acids. However, the downside is that they are very toxic, highly corrosive and poisonous on skin contact and when ingested or inhaled. Strong acids are available in both organic and inorganic form. These acids are also incorporated in several commercial cleaners. When using a strong acid, you will have to dilute it with water (50% to 80%) to reduce its concentration for safety purposes and to prevent it from corroding the joint or surface that you are working on.
Toxic Strong Inorganic Acids
- Hydrochloric acid (Muriatic acid)
- Sulphuric acid
- Hydroflouric acid
- Sodium hypochlorite
- Sodium bisulfate
Toxic Strong Organic Acids
- Oxalic acid
Commercial Penetrating Oils and Rust Removers
- EvapoRust (super safe rust remover)
- WD-40 (rust remover soak)
- Kano Kroil
- Liquid Wrench
- CLR – Calcium Lime Rust
- ZEP – Calcium Lime & Rust (stain remover)
- Loctite (rust remover) available in jelly form, has been used for more than 40 years
- Gasoila Free All Rust Spray
- CRC Knock-er Loose